Actually, deriving electrical energy from ocean waves, river currents or tidal currents has been done for some time now. Here's how one guy patented such a device:
http://www.gedwardcook.com/wave.html
Additionally, I think I'd disagree with
fo3nix concerning the source of the genesis of the wave movements. His/her statement would certainly be applicable to tides, but ocean, or lake waves are by generated by wind with the exception of
tsunamis of course, which are generated by earthquakes of one kind or another...