[Bre] dug up this excellent robot fish prototype project. The PPF-O9 has three servos. One on the forward fins to control depth, one on the middle joint, and one final one drives the tail fin. The battery box is mounted to the underside. The control scheme is interesting: the right stick controls left/right and up/down while the left stick controls the frequency and amplitude of the motion. They say the robot is fairly stable, but swimming and turning can be slow. They’ve included CAD files for almost every component to help you with your own designs.
In June, we highlighted a robofish designed for swarm communication.