A cool idea.
Someone should definitely consider doing this.
But I think a scuba-diver playermode and a underwater exploration map with a coral reef and such would be amazing.
Yeah, I definitely agree that some form of playertype that can behave normally (or at least not constantly float to the top) would be a better way to get around using environment water. It's much easier and more logical than re-working the environment water (although it'd be cool if this did get eventual updates, for things like waves and tides).
You could either have something similar to what Jakobblockhead described, where you move up/down through looking, or have it so your character is unaffected entirely by floating, and you either hold down (or tap) your crouch-button to swim down (in which case your character might as well just lay prone) or you can hold down (or tap) your spacebar in order to 'jump' and swim upwards.
The only issue with environment water is how much of a nuisance it is to keep your character in one single place.
And when doing so, you usually can't do much else, like using tools (like hammer/wrench) or typing, without letting go of crouch and floating upwards.
