encryptor:
Maybe the best idea would be to have a submarine with tube and a float where the BT module would be placed. I'll run the BT module's wires down the tube to the submarine. That way it can be BT radio controlled and still be a stealthy spy bot.
That strikes me as a far more practical idea from the technical point of view, you will still need to do some pretty good water proofing though.
Given the implication that you are planning to use this in a swimming pool to spy on people using the pool, I have to say that I would expect the swimmers and hotel management to take a very dim view of that. It might only be intended as a bit of fun, but in the wrong circumstances I think you could get into some serious trouble.