Here's what I remember from my dive class. You know those nifty waterproof disposable cameras? They are only good for 10 - 15 feet underwater. At that point the plastic begins to implode from the pressure. Even the $100-$150 so-called waterproof cameras have the same limitation. The only truely waterproof solution is a Nikon = many $$$
My point is, even though my Garmin SAYS it's waterproof, I really attribute that to rain-storm protection, not submersion protection.
Maybe there's some sort of clear dive-box of a thick material that would withstand pressure. No amount of zip-lock bagging will help at all below a few feet.
This sounds like a fun idea. If anyone has the solution, I'll be looking for it in the coming days.