This is a nice fish robot that swims. The model fish robot, PPF-09 has two joints (has three servos). It swims and turns with vibrations of a tail peduncle and tail fin. Also it can dive in the water with movement of the fins, which are located at the side of a head. They say the robot is fairly stable, but swimming and turning can be slow.
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.
The model fish robot, PPF-09 does not aim at a practical used underwater robot. Of course, the PPF-09 cannot work effectively in the sea.