This prototype robotic "fish" is actually modeled on a family of ancient marine reptiles known as Ichthyosaurus.
This robot has a single large motor driving its tail. There are two degrees-of-freedom (DOF) per front fin, which provide stability. The head rotates to aid orientation, and the jaw is actuated for animation purposes, giving the robot a total of seven DOF.
For those interested, detailed photos of the mechanism can be found here.