Starting this topic so that we can discuss potential house rules before adding them to the official list.
Adding the house rule regarding casting spells underwater.
I will elaborate on it a little on this, to have something more complete. The house rule would look like this:
Casting a spell underwater is possible, but with the following restrictions. Words are more difficult to form and somewhat distorted, so that the caster must take a -5 penalty just as if he were using Soft Words and an additional Botch die is added. Furthermore, Voice range magic is severely limited, normally reaching no farther than 5 paces.
Are we all in agreement?
Works for me
Yes.
Multicast spell mastery.
I tend to agree with the House Rule that requires taking this mastery once for each additional copy of the spell to be cast. So if a magus wants to be able to multicast three copies of a spell at once, he needs to take Multicast x2.
Are there objections to that?
I thought the idea was to stick to the core book as written, no more no less. Are we deviating from there? I myself am quite against spell mastery as a concept, but find it useful when using RAW (read: playing online or at conventions). If we go this route, I would vote for veto-ing spell mastery altogether. If not, I would leave it as it is in the RAW (+1 extra spell per Mastery level).
I don't much like that House Rule, myself. Granted, the multicast mastery is way more powerful than any other, but in my opinion that's more a case of the others being underpowered, given how much xp learning them cost (past the first two or three levels anyway). If someone puts 75 xp in a spell above and beyond the cost of learning the spell in the first place, he deserves to get something good ! (and I'm not just saying that because I have a spell with Mastery 3 myself 
).
What about Voice range spells if you have taken Quiet magic twice, how far do they reach? The description of Quiet magic says it's based on how loud the voice is not how articulate it is. The sound of a dolphin goes quite far underwater if we go by that.
Voice range depends on how far your voice projects. So if you whisper, your voice range can be a few centimetres only. If you cast silently you cannot use voice range at all.
Voice range for a dolphin can be kilometres.
You can still cast Voice Range spells on yourself according to the rules (effectively Personal Range)
As per the invitation post:
- Special Rules: House rules to be agreed upon by the troupe. Starting characters must be built using only the 5th Edition core rules.
I think spell mastery brings additional flavour to the system, but that the Multiple Casting master ability is overpowered compared to the others offered in the core book.
But this won't be a big deal in this saga, as combat won't be the focus of the game. So multiple casting will only come into play once in a very long while.
My objection to the Multiple Casting mastery ability, as it is written, is that it keeps growing at the same time as you add other abilities. None of the others work that way. EVen Quiet Casting, which provides benefits for taking it more than once, requires that it be taken twice.
No matter. Both you and Xavi are against the House Rule, so I won't go forward with it. But as another frequent poster on the board often says, what can be used by you can also be used against you. 
The effective range of a Voice spell is as far as your voice projects, as per ArM5 p.112. If you have Quiet Magic (either the virtue of the spell mastery ability) and use it, the range is reduced accordingly. If you cast silently, you can still use it at Personal range. (A case could be made for Touch range.)
If you are in animal shape, then the range of the animal's voice is used. But you still need to percieve the target, so a range of kilometers is of little help -- in period the concept of sonar doesn't exists, so dolphins perceive things underwater at a much more limited range (much like humans outside of water do). So your effective Voice range will be, at best, equivalent to Sight range. The deeper underwater, the lesser the range, since there is less light and visibility the deeper one goes.
Does that sound fair?
Another case could be made for uttering sounds unrelated to the incantation to carry the effect to the intended recipient (although I wouldn't allow it to go past 50 paces period, certainly not kilometers). See the Mute flaw on page 56.
Voice range can be significantly longer than 50 metres. Around 500 metres for booming voice is not unheard of. You might be coarse after a short while, though.
No biggie in any case with the Mastery abilities. I am seriously considering dropping them from the character, since they do not provide anything very interesting for me. I would leave them as they are, though. For other characters they are more interesting, specially for shapeshifter characters. Quiet and subtle magic are very useful in this format.
Xavi
I need them for my underwater explorer, since I decided not to take the Deft (Form) or Quiet Magic Virtues. Not even so much for the spells I already have, although I like the security of being able to cast Lungs of the Fish without words, but for the spells Pytheas will invent later they'll be essential.
I'm in favor of Arthur's multicasting house rule, but not in favor of ditching mastery altogether. I can live with multicasting as-is without problem, though.
1. Multiple Lab or Seasonal Activities
I am fairly flexible regarding this. If your magus wants to spend only part of the season in the lab (provided he can complete said activity within the season), or on different activities, I would allow the mixing based on the prorata of the lab totals or study totals. A penalty in days would be assigned to represent the reconfiguration of the lab if the type or Tech+Form combination changes. I am thinking 5 days, based on a season of 90 days.
2. Fixing an Arcane Connection
I would be inclined to assign a lab total required to perform this activity. I am thinking CrVi 5. Which means that as your labtotal rises, you would be able to fix more than one in a season.
3. Transfering Vis
The flip side of item 2, however, would be that I would also assign a lab total to the act of tranferring vis from a receptable to another. I am thinking ReVi 2 per pawn. So you could transfer quite a bit in a season, but that may negatively impact your other activities during the season.
All these points seem sensible to me. In fact I would like them to be official myself
Xavi
I like them too. Although point 3 will eventually become redundant since there is a level 10 ReVi guideline to move vis without a laboratory (the canon spell Gather the Essence of the Beast is more limited, but the guideline is quite clear that you can move vis from one object to another).
That is correct, although given the limited impact of doing it in the lab, some magi may never bother to learn the formulaic spell. The lab total to transfer from one receptacle to another is low enough that even an apprentice can tranfer a few pawns per season. It also helps keep vis sources secure, as tranferring the vis to a new receptacle severs the arcane connection to the source.