Just came across this news item. (From New Scientist website)...................

This video shows Japanese sailor Kenichi Horie setting off in a wave-powered boat that uses two spring-loaded fins to drive it forward using the sea's rocking motion. But powering boats like that is surely not this technology's best use.
Wind power is currently seeing a revival on large tankers to boost fuel efficiency. Perhaps Horie's power fins could make a similar contribution when combined with conventional power sources.
Link - New Scientist
http://environment.newscientist.com/article/dn13479-sailor-takes-on-pacific-in-wavepowered-boat.html
Video of prototype being tested.......
http://www1.suntory-mermaid2.com/english/movie/source.swf
Spring-powered flippers as a clean propulsion method (about 5 knots). Interesting.
Practical for small pleasure craft perhaps?