I like the N64 style, but what I'm most impressed with is the animation, it is so smooth specially for a gamejam. Doing that with transforms and the default Unity animator is incredible.
The movement on the water feels a little bit slow, although I think that's the way it is supposed to be as it closely resembles Mario's. My suggestion is to adjust either the speed or the air meter (make it go slower) so we can have more time to explore underwater. By looking at the pictures I guess there are hidden upgrades that allow that, but I couldn't find them :(
I know how hard it is to implement 3D movement, so choosing that for a gamejam makes me think you are kind of a daredevil :) Keep it up!