Vinay offers us some great leads on ocean thermal energy conversion (OTEC). OTEC uses the temperature differences between deep cold water and warm shallow water to create power. It's still more theory than practice, but the possibilities are pretty wild - including the use of "large, grazing plantships to produce hydrogen, ammonia, and methanol" or desalinization plants that'd combine energy production, clean water generation and fishfarming. Worth keeping an eye on....
To be fair, OTEC is a bit further along that this blog entry suggests: successful pilot plants in sevaral nations.
Have more details on what's actually been achived.
Talking about "wild".
There is also Marshall T. SavageŽs proposal to build self-contained extra-national floating cities ("Aquarius") around a core of OTEC-plants.
(Outlined in the book "The Millenial Project")
This would start with a ship-based OTEC used to generate the electricity for accretion of "seacrete" http://ww2.green-trust.org:8383/seacrete.htm to build the actual settlement.
The ideas from the book are carried on at the moment for example by http://www.luf.org. Lots of material there.
It's interesting to consider how this might affect ecosystems.