I'd like to bring this company, which got its start via Kickstarter, to the forum's attention. BlueRobotics
makes thrusters for marine ROVs. I'm using their T100 thrusters (image below) in the class I'm teaching. They are very sound performers, are reasonably priced, and do not corrode in salt water. They also sell reversible ESCs for the 3-phase brushless DC motors used in the thrusters, along with waterproof (IP67: 1 meter submersion) servos from Hitec. (The thrusters themselves have been tested to 3000 meters depth
(4500 p.s.i.) by Woods Hole Oceanographic Institute.)
Anyone who's done marine ROVs knows how limited the choices heretofore have been for thrusters: i.e. bilge pump motors with improvised props, Minn Kota electric trolling motors, or very expensive commercial ROV thrusters. BlueRobotics thrusters are filling a glaringly empty middle gap in this market.
The company has obviously adopted (perhaps without realizing it) the Parallax model for responsiveness to customer needs, and I can recommend them without reservation.