If you are using servo motors for any underwater activity, although these hitec waterproof servos
are super nice, you don't have to spend 50 dollars per servo. Here's how to waterproof servos yourself in a few steps. I havent tested this at super low depths, nor have I done extensive tests for long periods of time. However, I have made little robot fish that are totally fine after many swim sessions using this method.
Currently, I'm making a swimming robotic snake like this one
using this method.
I've read that people have successfully tried and used the condom method (wrap your servo motor in a condom, basically) but my condom broke : ) when I tried it.