In a sense, the spell has 3 effects:
It has either a ward against water or a PeAq effect or a very, very fancy MuAu effect (to create a sphere of air strong enough to repel an incredible amount of water (or other material?) while the interior remains normal air. Real air, it should be noted, does NOT repel water in this way. Even God created a 'firmament', which I suppose would work too, if one could figure out the proper Form for this, and in some sagas achieve a breakthrough.) A plain ReAq effect could work if the 'bubble' is created before entering the water, or in combination with a PeAq effect that created the initial bubble, or in combination with an effect that made the air neutrally buoyant in water, so that we don't have to worry about moving around masses of water. A continual PeAq effect is not desirable. Or, one could rule that if the mutated air is strong enough to keep water from rushing in, then nothing is needed to deal with the water, similar to creating a stone or fish in the water, and I think that's fair. At this point, no Aq effect is needed.
It has a CrAu effect, to create air that does not exist.
Effect #2 is pretty straightforward.
- ReAu, for the bubble to move with the caster. I'd add a +1 for complexity if the caster wants to be able to move the sphere of air as he wills.
Wards in AM5 are... weird. I would even say buggy, since it forms an important exception to the general rule that increasing the amount of stuff a spell effects increases the spell's level. For example, if I wanted to push a herd of stampeding elephants, I would need +2 for Group and then a few more magnitudes to account for the mass of elephants I wanted to push. But if I wanted to keep a herd of elephants from trampling me, the basic ward is sufficient. This would be ideal for our purposes, since we would no longer need to account for how much water we wanted to affect! But wards do not work at range, except when cast as a circle. A circle might work for an immovable bubble, but I don't think that is wanted.
So, if I wanted to be able to cast this anywhere, I think we are looking at:
(+2 for requisites, +1 for extra Mu effect, +1 or more for fancy effect)
Au lets the size be Ind, since Au is generous.
Creates a self-refreshing ball of air (CrAu) around the caster that is neutrally buoyant (Mu1), that is strong enough to repel Aq (Mu2) but acts as normal air (not too thick to move in, be comfortable, breathe, etc. repel anything within, for fancy1), that moves with the caster (Re1) [and can be moved by the caster if he concentrates (fancy+1)]. Add more fancies for complexity, as you fancy; +1 to +3 would not be wrong. Breathing this air exclusively for too long can cause Warping, and it is a magical medium so it needs to Penetrate. (Yes, if you CrAu, the air needs to Penetrate! Woohoo !) The base level is the worst of Cr/Mu/Re.
Other ways to do this. Mu(Re)Aq(Au) with some added magnitudes for complexity works too. But you have to already be in the water.
(I'm assuming that magi will not suffocate on Creoed air or water Mutoed into air!)