This fish from the University of Essex is the worlds most advanced, fully autonomous, robotic fish. It swims, dives, and avoids obstacles on it's own and has some pretty amazing specs for speed, acceleration, and turning ability. For this and other demonstrations it has actually been speed limited to conserve battery power.

For those interested in how this came to be, take a look at this large but VERY informative video of the design process behind RoboCarp.

This fish is not the only robotic fish out there. Robotic-fish.net keeps a listing of a large number of them here. These include a robotic coelacanth and manta ray. The forum Ornithopter Zone has numerous home made RC fish as well if you search the forum.

I think these things are totally cool. First, the engineering behind them is impressive, and second, the mechanics have real potential to improve the efficiency of any water based vehicle. An example of this crossover from animal biomechanics to engineering is the Hobie Mirage kayak that uses two fin like sweeps to generate pedal power. The drive system is apparently uses a similar design as a penguin wing.
The video demonstrates the effeciency and power of the Mirage fairly well. If you want, take a look at this video that shows a Mirage beating one world champion paddler and holding it's own against two world champion paddlers.
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(Sorry for the promotional nature of the vids, it's all I could find. I promise I'm not shilling for boat manufacturers on this site, it's been years since I paddled a kayak and I have never pedaled one)