I think this area is called Keystone. We're in the reef just offshore...which means you're not hallucinating the water or your new form, if you were worried about that.

[The shark wiggles against his side, and he turns so that it can't bump into his dorsal fins by accident. The shark responds by letting go of his hand, so it can get a better grip further up his arm.]

The unstable magic here is changing everyone who ends up submerged in the water into forms suited to it.