I was considering installing this effect in an item, but can't decide on what the appropriate Arts would be.
MuAq (to change the water so that it allows transmission of speech)
MuCo (to change the caster so that they can speak underwater)
MuIm(Aq) (to change the caster's voice so that it transmits underwater)
I would prefer MuAq (Aquam specialist) but am open to your ideas!
, V
Two observations:
To cast underwater without penalty, you need not only something allowing speech, but also something allowing unimpeded movement. Otherwise, I'd only allow spells to be cast with Subtle gestures or No gestures -- it's difficult to produce Bold gestures underwater.
You can often achieve the same result with different Art combinations. If you are looking for an enchanted item, and it's not a talisman, something that affects the environment is much better than something that affects you, since in the latter case you have to worry about penetration.
I can see several ways of doing this using Aquam.
If you just want to be able to speak, I'd probably use Rego Aquam to make the water around the bearer's head trap some air when the wearer dives, or if the wearer blows out some air while submerged. This is best enchanted into an item like a necklace or a haircomb that will be in the affected area. I'd use the Base 1 guideline, "control water in an extremely gentle way" (all it needs to do is keep some air trapped), with +1 magnitude for a slightly unnatural effect. Thus, Base 2, +1 Touch, +1 Part (much safer than Ind if the water phenomenon you are immersed in is too large to fit into a Base Individual), +1Conc gives level 5; to which you should probably add 5 levels to have the item maintain concentration, and a certain number of levels for how often you want to use the effect.
If you also want to gesture freely, it gets more complicated. I'd probably use Muto Aquam to change the water around the wearer into an unnatural liquid that, although buoyant like water, carries sound like air and offers the same resistance to gestures. This is Base 4 rather than Base 2, +1Touch, +1Part, +1Conc -- yielding a level 15 effect to which you should probably add 5 levels to have the item maintain concentration, and a certain number of levels for how often you want to use the effect (basically, it's 2 magnitudes higher than the previous one). You'd also need some means of propulsion, possibly a fish (or a grog!) that you can ride -- remember that the water around you parts as easily as air, so you create very little drag).
Personally, I'd just do the following. Forget about the magic item, and accept the subtle gestures -- it's a meager -2 penalty. Cast on yourself a MuCo level 4 spell that allows you to speak underwater (Base 2 "grant a minor ability", +2 Sun). Outside an Aura, you only need a Stamina+Muto+Corpus total of 20 to cast it spontaneously without fatigue or risk of botches; in fact, a total of 18 if you are willing to use exaggerated gestures and a booming voice (and even less with an appropriate bonus from your Talisman, or if you are willing to spend 15 minutes for ceremonial casting). If you can reach this level by spending a season or two to improve your Muto or your Corpus, it's probably worth it. Push come to shove, you can easily learn it as a formulaic spell, possibly in combination with some other useful MuCo effect (e.g. some Disguise spell, or some spell making you very large or very small, or some spell giving you other useful abilities like Eyes of the Cat). Or if you only have to cast it once or twice, just cast it as a fatiguing spontaneous spell (extending it to Moon duration still makes it only level 5).
An alternative? A magical scuba mask! The principle of diving with an air sac over your head was known to the Romans and Greeks (we have a fresco from the time of Aristotle depicting the technique). What your magus then needs is just a bowl maybe a hand in diameter, pressed against his lower face, and inscribed with a Chamber of the Spring Breezes spell with Circle Target and Ring Duration -- to keep the air within fresh. The effect level is just 5, and you can cast it spontaneously (or have some friendly Creo Auram magus do it for you, probably without fatigue) and keep it always with you. It works for your grogs too!
Magi of Hermes discusses this a bit. Without magical augmentation it suggests applying penalties equal to no words and subtle gestures. Even with augmentation that would allow standard voice, the magus is unable to target anything beyond the surface of the water.
There are some underwater spells in the Hermanus chapter of Legends as well.
For an alternative, turn yourself into a merman. No speech or mobility issues anymore. Presteris, our salty water flambeau did that.
I'd disagree about the gestures part. A merman is slowed by the water like everyone else.
And what would be the effect level for turning into a merman?
Hmm, I had missed this part. However, I think it's really running agains the corebook. It says that the magus cannot target anything beyond the surface of the water withVoicerange spells, because sound does not carry. I think this violates the corebook rules that state that for Voice range it does not matter if sound is blocked around the targer or not -- for example, a PeIm spell cancelling all sound that reaches a certain area does not prevent magic from affecting that area.
Still, it's very hard to perceive something above the water's surface while submerged, so I'd agree with the fact that targeting something without Arcane Connections or magical assistance seems very hard to do -- but for a different reason.
The same chapter also presents a spell that transforms the caster's voice "into a loud high-pitched click" so that it carries far in water. Again, this runs explicitly against the corebook description of Voice range, that says that magical enhancements that transform the wearer's Voice do not increase the Range. Bah! A lot of the spells in Magi of Hermes are really dubious.
FTFY
Edit: oh, yeah, forgot the smileys. 
Sadly, MoH is extra bad.
There is also an existing spell in HoH:MC (Bjornaer section) to help overcome limited voice/gestures.
I think they exist in almost every book, and we who discuss it are quite pedantic. For example Covenants has Eternal Repitition in a Bottomless Pool, which could potentially allow two people to benefit from the same book simultaneously.
Hermetic Projects has Conjuration of the Seaworthy Cog, which uses the Target:Structure incorrectly, but I believe the level ends up the same, when adjusting it to T:Ind and increasing the size to an appropriate multiple.
I don't see why this is a problem? Infact I bought an item with this effect for Titus specifically for that purpose. It's all according to the rules. That's not really problematic. Spells and effects that do not, are.
Structure is infact the most appropriate Target for that spell, as Structure specifically includes the structure itself (as opposed to Room), and a cog consists of multiple obejcts.
Two people learning from a book is a problem, in fact it completely invalidates the teaching method for Hermetic Arts, as Hermetic Arts can only be taught one on one. In my opinion, the device is fine, so long as someone isn't actually using the book. If the book is in use the device behaves erratically, because it won't change pages at the pace of the user of the mirror, among other things.
So, no. It's not a valid target for creating a ship from scratch.
Except that ArM5 p.113 (the top insert) specifically says "The target of a Creo spell that actually created something is the thing created. The target is thus always Individual or Group."
You need an existing structure to use T:Struct.
I still fail to see the problem.
Teaching is still extremely good, simply because the source quality is so high.
For a single student, it's 9+(Teacher's)Communication+(Teacher's) Teaching (potentially +3 from a well-designed lab).
Typically very high compared to books, with the exception of a few low-level summae.
Consider Tria of Bonisagus, Communication 5, Good Teacher.
She writes books to Q: 14 - highest possible (not using covenants) or 17 (using covenants).
When writing summae she can sacrifice levels for quality, like everyone else.
If she has a teaching score of 1, she teaches for Q: 20 (not using a lab designed for it) or Q: 23 (with a lab designed for it).
If she can be bothered to care this much, she's lkely to have a higher score in teachng, meaning her quality as a source is even better.
Having a mirror that allows me to read a book via AC (that is, via Endless Repetiton) merely equals having an extra copy of a given book, or does scribes unbalance things as well? Because being able to read your own books via the Endless Repetitions is less useful than a scribe - with a scribe you can sell the book afterwards.
And as for the readers not turning the pages at the same rate, that's just absurd.
Sorry, but the spell is designed to scry the pages in the book, you can use it to read a closed book locked in a chest!
That's what the spell is for! It would need ReAn to turn the pages if that was a parameter!
I'm sorry, I really don't see neither your point nor your argument.
So, no. It's not a valid target for creating a ship from scratch.
[/quote]
Good point.
Duplicate.
Did I say scribes imbalance things? Don't change the argument, but I'll address this later, and demonstrate how I don't find scribing multiple copies of a book to be equal to this spell.
My problem with it is that it is a violation of mechanics. It becomes possible to imbue a device with this power, indeed multiple devices, and then just hand it out to a group of people, and boom, they all read from the best available SQ available for whatever they want. That, IMO, breaks the setting, it essentially makes the idea of learning Hermetic Arts a very mechanical process that can be mass-produced with these devices, whereas before it was only one-on-one. One teacher and a student, or one book and a reader. Now this spell is one book, and as many readers as can manage the effect, so long as they have an AC. This makes for opportunities of perfect advancement. Ever had a character who wanted to read a book, but another character had it first? Ever had a character that had plans to read a book, and then something came up? Those kinds of things lead to interesting story possibilities. The spell, effectively gets rid of that problem, IMO, it's setting breaking.
As far as the absurdity of a user mystically interfering with the spell, that was a mechanic I used in Bibracte, because the majority of the troupe found it unacceptable. And Bibracte is pretty high fantasy.
As to scribing another copy of the book, resources must be acquired to do that, sufficient parchment, a bookbinder, perhaps an illuminator, and a magus must have the ability to scribe the book, and then spend the necessary seasons to do so, investing time in the process to end up with an added additional resource. That isn't at all close to the lack of effort when that Endless Repetition is introduced into a saga. It's a spell that seems like it makes a lot of sense, but it allows for an almost perfect advancement plan for the magus wielding it, and I really don't like that, because I like the interplay between characters over covenant resources.
Probably right. Just assume that the slow gestures are still performed in a round. It is 6 seconds long after all 
Yep, the spell is higher level, but he was half merman himself, so no biggie for him. Just an alternative: instead of fighting against a hostile environment, become aligned to it. Harder, but it has other uses as well
Cheers,
Xavi
Thread-irrelevant discussion moved to more relevant location.