The problem is that whatever 'natural' route is taken for power you are sucking energy from the ecosystem. That can't be good! There's the recent example I read about somewhere in Switzerland where a geo-thermal plant cooled the rock around it down so much that it caused earth tremours. They had to shut it down.

If you suck loads of energy from the oceans or the wind what is the greater effect on things like the gulf stream? Kill the gulf stream and The British Isles will be like Scandinavia.