Just do a google search on "robot fish" and you'll get several web pages to look at. This one has a link on the right side to watch a little movie. It does swim realistically. Maybe we can eventually replace all of our live fish with versions of these - no more cycling, no more fish diseases, just plastic plastic plants, robot fish, and tap water! How cool is that? NOT! It just wouldn't be the same would it?
http://news.bbc.co.uk/2/hi/science/nature/4313266.stm#
But, it's a pretty cool robot fish!