Great project- My ROV, “Bob” is based on the “Sea Fox”. It uses 5, 500 gpm Johnson Bilge Pumps ($10 Each) for thrusters. Unfortunately these pumps have increased in cost to $20 since I built Bob in 2002. If you have more time than money you can convert bilge pumps to use propellers similar to a boat and then reverse wire the so you can reverse them. Since each motor can produce forward and reverse thrust you can design and ROV with only 3 motors. The control is an old Atari switch type joy stick ($10 on ebay), that now houses a 9 volt battery, and is wired to mechanical relays on the ROV using Cat-5 (Standard 4-pair Computer Network) cable. An extension cord provides power to the ROV from a 12volt trolling motor battery”. [via] Link.
HOW TO Build Your Own Underwater ROV