Now, boat owners need not feel bad at all about using fossil fuel. I know they have been in the sights of the green community because the bulk of the world's fossil fuels are used by boats and ships. Now, there is a technical solution that should make them feel less guilty: http://www.dsehybrid.com/ A hybrid solar-diesel-electric boat. There are also rumors that the Japanese and the Norwegians are independently working on large oceangoing vessels powered by solar power alone.
And I have written about this before but the Dutch Czeers company makes a solar powered speedboat: http://www.czeers.com/ It would be inappropriate not to mention them on this post.
Cheers! And happy green boating!