If you happen to visit the Spanish port of Gijon, you may notice some giant yellow robotic fish swimming around. These 5 foot long swimmers are part of a proposed sensor network to detect pollutants in the water. Equipped with an array of sensors, the fish can test for general water quality, or swap out the sensors for specific testing. They communicate with each other to keep from straying too far from the rest of the network and the base charging station.
The fish was designed by the Shoal Consortium, a European commission funded program that draws from intelligent minds in universities all over europe. While the fish cost over $35,000 right now, mass production should reduce that cost considerably.
You can see them swimming around in the BBC video at the link.