As of right now, I haven't quite figured it out for underwater, as it would be quite hard to waterproof the entire motor. As of right now, I have thought of 2 solutions. You can either buy a RC boat motor, like this one (https://hobbyking.com/en_us/turnigy-xk2430-5300kv-brushless-inrunner.html), which as far as I know is waterproof. Otherwise, you can make your own generator by winding your own coils and making your own motor. This will be waterproof if you use insulated wire in the coil. I am working on this right now, and will try and get back to you about it.