Ah! Great idea. Keep it.
Chris
I'd have thought it'd do something underwater, although you're probably right that the effect of a sudden current might well not be the same as being hit by a jet of water in air. I'm struggling to work out how the effect of the increased resistance to the jet (presumably slowing it) would interact with the presumable increase in difficulty in not getting knockled back due to swimming rather than having good footing.
What technique are you considering an electric eel fall under? Perdo?
Useful spells:
Warmth of a Summer's Morn ReIg(Co) 10 (Base 4, +2 sun)
Maintains the heat of the caster's body at a constant level. The spell will fail under extreme external conditions (levels of Ignem doing more than +5 damage).
I'm not sure whether 4 is the right base level for this spell - the spell guidelines for ReIg focus exclusively on fire rather than heat,so I've gone for a base of 4 - "control a fire in a slightly unnatural fashion". (Versions of) this spell must exist somewhere else already, though, surely?
I think a variant of "Push of the Gentle Wave" should work as a propulsion system (possibly touch rather than voice)?
That does sound useful - I'll keep an eye out for the book, although I probably won't be able to acquire it any time in the immediate future. What was the pressure handling spell based off? The MuCo base 2 guideline to grant an ability, or something using aquam?
Going from Fae Blood to Mythic Blood is (imo) a very good decision. Not only will it be more helpful for what you want to do, but adding in all the faerie stuff was (imo) muddling things up. Now your character will be more focused and will probably help you make it more interesting and fun.
On your note about how you didn't want to use something like Greater Immunity: Deprivation, so your character didn't have to breath underwater at all, I don't think you should skip that one because you don't want it to be 'too easy' to live underwater. It's very, very easy to create a breath-underwater spell (arguably, you could do it with MuCo level 4, base 2+2 for sun)... it's not so easy to deal with any problems that could be had underwater. For example, setting up a functional lab, or dealing with any underwater enemies that you could 'buy' through a flaw (supernatural nuisance, supernatural enemies, etc.). That stuff is the interesting stuff... breathing water is the simple, easy, not-so-interesting thing.
On your note about having 4 more virtue points (you want) to spend: I don't know how exactly you'd want to use the Mythic Blood freebie spell, but also consider the fact that in the True Lineages book, if you have Mythic Blood, Heroic Virtues are opened up to you. Of particular interest would be Heroic Birthright, which would allow you to build similar 'free' spells to the Mythic Blood freebie, with 15 spell level points. So you could use the Mythic Blood spell for a different purpose (maybe an underwater travel spell?) and use the Heroic Birthright virtue to create an instant-cast spell to breath underwater. You could also think about adding Cautious Sorcerer, because I'd imagine there's going to be a few extra botch dice thrown in when casting all those spells underwater... I'd imagine any underwater character would learn to be very, very careful about how they cast spells if they're going to be casting them underwater at all. LOL. You could also think about Deft Form (Aquam), which would definitely be in the flavor of your character, while also helping solve the problem of casting underwater with less penalties. A Deft Form Aquam + 1 rank of Quiet Magic would allow you to cast Aquam with no penalty underwater, while casting other forms with less penalty underwater -- which would suit the nature of such a character very well.
On your note about "ways of the deep" -- I actually like that, but you could simply go with "Ways of the Water's Below" -- ie water that's below the surface. That's plenty narrow for a major virtue, especially given the nature of your character.
Electric eels are exclusively South American, despite what it says in books for previous editions of ArsM 
However, the electric ray (crampfish, numbfish or Torpedo) is found in the North Atlantic and the Mediterranean, and mentioned by Pliny
I would make this a Rego Animal or Rego Corpus effect, to paralyse the prey, rather than linking it to electricity which was unknown at the time.
Mark
Surviving the Deep(MoH pg 76)
MuCo30
R Touch, D: Sun, T: Ind
This effect strengthens the body, increasing its mass and allowing it to resist the great pressures of the deep ocean, as well as providing +3 to soak.
Thank-you.
Hmmm. I think this build is going to need rather more muto and corpus than I had originally envisaged.
Actually, if you do not care about the +3 bonus to soak, it's just a MuCo 5 spell.
Base 2 (change someone to give them a minor ability) + 1 Touch + 2 Sun. Like eyes of the cat.
Also, I'm not sure that "pressure increases with depth" is true in Aristotelian physics.
EDIT: In fact, I'm fairly certain it is not true! It was Archimedes that came up with the idea, a hundred years after Aristotle. Had Aristotle understood Archimedes' theory (which is basically in agreement with modern physics) he would not have been able to incorrectly conclude that air is weightless - as he did - from an experiment where he found that an airtight sac had the same weight whether empty or full of air.
I think "minor ability" is a bit on the low side. Base 4-5 feels like a better range i would say.
How did he make it empty? (yes, serious question)
No but you can probably argue that it is at least "known but not understood" by some(few) at the time(ie. making it basically up to the SG how to handle it).
Diving and underwater salvaging isnt a new thing, but although details on methods and knowledge of the time is all too scarce(to say the least) it was known as a very dangerous line of work(the preferred method was to simply drop a hook on a rope and wish for luck, as this avoided any diving at all).
Much better range.
I'm not sure what makes people think that water-breathing is a "minor ability". Even if the comparison were "changing eyes to a cat's vs. changing lungs to gills" were not huge enough - and that's not the comparison - that's not the point. It's "a minor ability" - seeing in the dark like a cat is much more minor than breathing water like a fish, which strikes me as fairly major. (And a shame that fansite doesn't insist on the R/D/T design notes, or have editors who care about such. As a result, it has all the tripe one would expect from "fans" mixed in with the good. 
)
If you're going to do something, do it right, and make the spell more useful. This is not something you want to have to Spont anyway.Cloak of Fresh Air
ReAu(Aq) 20
Forms a bubble of the existing air around the caster. Can work under water, or out of water against gas, smoke, smells, or other air-borne threats if the original air is good to breath. Allows talking, and keeps the caster dry against water, or even rain/etc. Can also hold air around others or objects, and/or create a balloon-like, moveable globe-sample of a captured gas.
(Base 2, +1 Touch, +2 Sun, +1 Part, +1 Re Req, +1 complexity)
Note that Base 1 ReAq is "control a liquid in a gentle way" - which would work, but it's lower than the Base 2 ReAu - so we have to use the larger (and that makes this ReAu(Aq) instead of ReAq(Au) ). But it also means the "bubble" is stronger than just "gentle", and that's not a bad thing either if the water is more agitated than just a calm pond.
The +1 complexity is a hand-wave blanket to cover the wide flexibility of this effect. Adjust to fit your Troupe/Saga as desired.
I'd also suggest a "See through water" spell, and, again, for my money, combining that with a "see through air effects" as an InAq(Au) is the way to go. It would be more than a little ironic if he were on a boat in a bad storm or fog and couldn't see the reef until the boat sank. 
This is one of the reasons that the "medieval paradigm" concept was shuffled out of 5th ed., that "scholars" often know far less of the real world from their "expert sources" than peasants who deal with something every day.
Folded it flat?
More precisely, Aristotle wondered whether air had weight (this is actually a very profound question to ask for someone who lacks the modern scientific background). So he devised an experiment to test for the answer (a very modern approach).
He started with an empty animal bladder.
He measured its weight.
He then blew air into it (like a baloon) and tied it shut.
He measured the weight of the bladder with the air he had blown inside.
He found that the two weights coincided.
He concluded that since weight of bladder = weight of bladder + air, then air must be weightless.
While this is wrong, I really think it is still a testimony to Aristotle's brilliancy.
And then what?...
DW's point, I believe, is that pulling a vacuum - even the concept of a vacuum, of something "empty of air" - is not very medieval, even with magic involved.
You have to think of something before you can experiment with it. Not impossible, but an explanation is desirable. Hence the question. 
Well, the inside of the "empty" bladder was certainly not perfect vacuum as we understand it, but it's pretty obvious - both to us and to Aristotle - that if you take the "empty" bladder and blow into so that it bulges like a baloon, it now contains more air than it did previously.
I was wondering how this could be, but some quick reading seems to indicate ideas from one and a half millenia earlier were lost for quite a while. I figured since the use of a vacuum was around since Ctesibius's time, the concept should be around in medieval times, even if not among the common folk. Guess maybe not. How sad.
Chris
The concept is around yeah, but the ability to create that concept in reality? Now thats a VERY different thing.
Of course, with magi loose in the world this wouldnt be an issue, just a quick ReAu or PeAu spell and you have your vacuum. And no Cuch, vacuum as an idea existed(exactly how widespread is probably an open question, but IIRC the first known attempt at thinking up an engine based on utilising vacuum came no later than the 14th or 15th century). The main point was that noone had the ability to achieve anything remotely near vacuum at the time.
Add to that the problem with knowing when someone is using an idea as a metaphorical or philosophical description rather than as an attempt to accurately describe something scientifically...
This is one of the main reasons i run with real physics and just let the contemporary ideas be what they really were, attempts at describing and modelling those same real physics. And to describe those attempts to other contemporaries.
Which is about 2 centuries late. And there is a marked diff between what you're describing, and understanding the concept of a "vacuum" - the fact that water or air rushes in as a plunger is drawn away and getting power from that is not the same.
I'd be interested to hear what your references are, medieval or classical, and whether they are actually talking about the concept of a true "vacuum" or merely observational action/reaction.
Eh, the CONCEPT is described reasonably accurately by Heron of Alexandria.
Well, not online, but it only took me a few tries to find a site which quotes Heron´s above mentioned description.
cosmovisions.com/Williams010907.htm
"Every body," he tells us, "is composed of minute particles, between which are empty spaces less than these particles of the body. It is, therefore, erroneous to say that there is no vacuum except by the application of force, and that every space is full either of air or water or some other substance. But in proportion as any one of these particles recedes, some other follows it and fills the vacant space; therefore there is no continuous vacuum, except by the application of some force [like suction] - that is to say, an absolute vacuum is never found, except as it is produced artificially."
But i would have to read up again to be able to give you a more serious set of sources(simply said im very fuzzy about the details here because its far too long since i actively read anything about it).
Wiki gives a few hints overall:
en.wikipedia.org/wiki/Vacuum#His ... rpretation
Not finding the reference for the earliest attempt at vacuum engine though, Papin in 17th century is better known due to his influence on steam engine construction and comes up in all searches instead.
The comment on this book might be relevant:
scientificblogging.com/news_ ... scientists
The intellectual history of the Middle Ages will always be somewhat subjective. If you want to write a book, you pick a point of view and the data you want to include. Hannam seeks to show how the scientists of fourteenth-century Paris and Oxford discussed the rotation of the earth, the properties of a vacuum, uniform acceleration and relative motion.
I've been trying to think of a suitable ancestor for the Mythic Blood, and struggling slightly. The rules require that "you are a blood descendant of either a wizard of legend (possibly one of the Twelve founders, or some other ancient and powerful sorceror) or a supernatural being (such as a dragon)". None of the founders seem obvious choices. Ancient and powerful sorcerors I'm also struggling on a bit, but have come up with a few ideas:
Nimue/Vivianne is the most obvious choice, possibly too obvious. She does have the advantage that I'd actually heard of her before starting looking into possible mythological ancestors. In the arthurian legends she gave Arthur Excalibur but also entraps Merlin after he becomes enamoured of her. I believe the Stonehengle tribunal book was entitled "Heirs to Merlin" - is it worth trying to find it for backstory? Nimue also has the advantage that she's a relatively obvious ancestor to be trying to emulate in some way.
Potential minor personality flaws: Continence or Ambitious, depending on quite how you interpret her entrapment of Merlin. Continence has an obvious problem in connection to being her descendant, although probably not an insurmountable one. Higher purpose (with some purpose relating to the good rulership of Britain), depending on how you interpret her assistance to Arthur. That might also fit in well with a Visions supernatural flaw, and possibly a delusion. Also having to be very careful to avoid ending up before a tribunal in short order, of course. Other possible things to hang off the ancestry might be "Unaging" and Blessing/Curse of Venus.
Scyllias and his daughter Cyana. Not actually magicians according to standard tales, although they had a number of tales attributed to them that wouldn't have been possible without magic. They were Greek divers who worked to retrieve treasures from the seafloor for the Persian king Xerxes. When Xerxes wouldn't allow them to return to their homeland, they swam underwater to cut the anchors of his ships, which were then driven onto the rocks.
Probably dubious as mythic ancestors. No very obvious personality traits, although something involving vengefulness might fit.
[*]Thessaloniki was a half-sister of Alexander the Great. After his death, she threw herself into the sea in grief, but did not drown. Rather, she transformed into a mermaid. Sailors upon the Mediterranean whom she encountered would be asked "Is Alexander the king alive?". If they answered that yes, he lived and was king, she would ensure they had calm passage. If they answered in any other manner, she would be transformed by rage, and invoke mighty storms to send the ship and sailors to the seafloor.
Possible associated minor personality flaws: wrathful.
Both of the latter two cases don't really seem to fit the bill as mythic sorcerors. Nimue has her points, but, as with faerie blood, would seem to distract from the core concept quite a bit (or, admittedly, create a more fleshed out character). Does anyone have any better ideas for sorcerors? I also haven't really explored the idea of a supernatural being.
On another matter, I'd been considering the problem of visitors. Whilst there's undoubtedly something to be said for being able to work undisturbed, sometimes it's useful for the Redcaps to be able to reach you/ not have a good excuse for not reaching you. Does the following device work?
Cassus Campana
On a small promentary into the lake within which Aequora dwells hangs an empty bell - its clapper removed. If someone attempts to ring the bell regardless, Aequora may choose to surface from the waters shortly afterwards. The clapper from the bell hangs outside her sunken laboratory, and upon the bell being "rung" a single clear note chimes from it.
Lesser enchanted device, CrIm 5 (Base 1, +4 arcane connection). The bell is an arcane connection to the clapper (rather a inverted perspective, but I can't see any reason it's not), and "ringing the bell" is the trigger for the effect. In practice, I'd probably want to add some uses per day.
Does that work?
The rules don't require that you know who your ancestor is, though.
But there are endless stories of sea monsters or river monsters forcing themselves on one princess or another, and some prodigy emerges as the progeny. And endless minor gods and figures who were from rivers or lakes or whatever - invent something local.
Eppona was a horse goddess but also goddess of the seas - Irish?
If you have RoP:M then you could look at the Atlantian Magic virtue. Gives you Range of Water-way, Duration: Storm, and Target: Body-of-Water.
I actually think they're fine for ancestors that would give you Mythic Blood. Remember, these "ancestors" wouldn't have to be magi themselves, just supernatural. Furthermore, you wouldn't have to be directly descended from them -- it could be in many ways distant. You can also be relatively flexible with whatever personality traits you associate with the mythic ancestors, because we don't know much about even their legends to begin with (in some versions, Nimeuh is the lady of the lake, in others she's sister to Morgana, in some versions, she's sister to Arthur, etc.). Be creative and make it work for both you and your character. These are all good ideas though -- that you're taking them from actual pieces of fiction and/or legend does, I think, add to the flavor of your character -- even if you use poetic license to describe the mythic ancestor.