Nature is full of great examples of highly efficient mechanical systems, and roboticists are keen to capitalize on those designs. When it comes to underwater vehicles, fish are a popular animal to emulate.
MIT's RoboTuna, developed in the early 90s, has spawned (ha!) two spinoff robotic fish projects. One we've talked about before: GhostSwimmer. This project, led by Boston Engineering, has evolved. The previous demonstration videos showed the robot propelled by a tail propeller but directed by the movement of the "fin"; the new version is actually propelled by the tail fin motion. Check it out here:
Next up is a newer set of fish from MIT. The two different types of fish mimic two different types of swimming mecahnics in fish like trout versus fish like sharks. MIT's news release says "the new robotic fish, each less than a foot long, are powered by a single motor and are made of fewer than 10 individual components."
Soon I'm sure we'll have even more roboticists swimming with the fishes!