A cool touch would've been to detect when the player is underwater and use a shader to color the entire screen slightly blue. That way you woulld get more of a feeling of being underwater.
There actually is a shader doing that. :) I have a blue fog shader that I'm using for everything under the water. Maybe I need to adjust it to make it more obvious.
It could be that the shader doesn't work in the html5 version, we ran into the exact same issue in our game. Some shaders won't show when playing in browser, only simple ones do.
