It's been years since I worked on autonomous submarines, but underwater robotics still turn my crank. Gizmodo has an article on work at the University of Washington carried out in the lab of Kristi Morgansen. I am particularly keen on the design of their robot; the pectoral fins are actuated by microlite servos, the rear of the fish is activated by a high speed servo, and the tail fin is driven by a high-torque micro motor.
This sort of design should yield higher maneuverability, and greater mechanical efficiency through reduced cavitation and drag. Of course fish have already proven the superiority of this sort of an arrangement! A useful contribution of their research is that they've derived a general mathematical framework for controlling robots that propel themselves by changing their shape.
Their current tests involve trying to make the fish school using very simple communication, and very simple commands, such as "swim in the same direction as me." The theoretical and practical groundwork for this flocking, schooling, and herding was laid by Craig Reynolds back in 1987. It will be cool to see a physical embodiment of Boids!