The craft needs to have some kind of manipulator so it can interact with its underwater environment. That means I needed some kind of servo which will operate underwater. I chose to use the HiTec HS-5646WP and its analog sibling. There are a few waterproof servos available but I chose HiTec because it has a great reputation. The servos themselves are only IP67 rated so theoretically they're only good to about 1 meter, which isn't very much for an ROV. I hear chatter that they're good at much greater depths, so we'll see how they survive. I wouldn't be at all surprised if I end up rebuilding them (something like this by the Society of Robots perhaps).