I don't think it can work. Hmm, with Cr(Re)Au you should be able to create breathable air continuously in your immediate vicinity.
The bigger problem I see is that this does not keep water out of the bubble, i.e., breathing might still be impossible. If you could create a scuba-like mask that keeps water out from the face, then with just Creo Auram this would be feasible.
Otherwise, I'd think you'd have to create a small and powerful tornado to be able to push shallow water away from you, and that would create other problems!
Another related problem is visibility, I think, as creating a continuous supply of air would create lots of bubble/turbulence. Which you could see through with a InAu spont, right? So not as big a deal.
My personal mistral Cr(Re)Au 5 Base 1, +1 Touch, + 1 Diameter, +2 Unnatural, creates a constant breeze just around the target.
Water, water, everywhere but here ReAq 10: Base 5 R:P, D: Diameter + 1, T:Ind
Let's not forget Clear Sight of the Naiad InAq 5, InAq10 for D: Diam.
and True Sight of the Air InAu 15.
Unfortunately Scylax is weak with Aquam. He could spont to destroy the flow of water into the cave and then spont to destroy the liquid, but this would require some good rolls, punctuated by fatigue recovery, and the possible destruction of the cave's resources, which he is very unwilling to even contemplate
Much easier is to gain a fish's gills, right? Not sure about the medieval beliefs around that one, but based on Eyes of the Cat, MuCo(An) Base 3 + R:P+ 2 Sun Duration, req. free ...of course, this might leave one unable to breathe except underwater! Scylax could spont this one reasonably well. But it also might be higher in level than I give it credit for.