Why are sailors against offshore windfarms? Do you mean recreational sailors, or professional?
Tidal power has worked in places like France, where there is a barrage accross the Rance akin to the one that has been mooted over the river Severn. I think the difference here is that we've been talking about different types of tidal generator - more akin to underwater wind turbines.
Is the marine life problem so insoluble? I would have thought that some carefully placed nets could stop the basking sharks from being sliced and diced. I'd be interested to hear more about it but didn't find much online (not that I looked hard so far).
There used to be a lot of fuss about wind turbines hitting birds, but I understand it's not as big an issue as was originally feared.