If this was completely without any obstacles, it would have been brought to market about 50 years ago. There are obstacles for sure, but they don't sound insurmountable to me.
Salt. Salt is a problem for anything that floats on the ocean. Plenty of things do that, it's not such a deal breaker.
Bird poop. You need to either be close to some islands, or in the middle of some migration line. In the Pacific ocean there are millions of square miles that don't qualify for either of that.
Barnacles, mollusks, etc. Maybe you can get away with some special paints, or with periodic cleanup. Mauybe you need to coat the bottom of the floating structure with some film of pesticide-infused plastic. I don't know. But I'm willing to give this idea a chance to succeed. What are the alternatives? Floating nuclear power plants?
Assuming that the goal is a reduction in atmospheric CO2, there are many alternatives that are at least an order of magnitude more cost-effective. Of those, more land based wind and solar plants are for now the best. The cost of land generally is almost negligibly small - about 0.4% of total for a recent project I looked at. Admittedly, this was in Australia, but given that solar power stations cost circa $0.5 million per hectare in equipment and installation, land (rural land of course) would need to be pretty expensive before it became a significant factor.
Salt. Salt makes operating things in the seas an absolute maintenance nightmare.