I'm sorry, but even reverse osmosis plants use chemicals to prep and clean the water and to protect their equipment. Sea water is astoundingly corrosive, so if you don't want to be replacing all the core parts of your plant every year you have to treat it to prevent that damage. Those chemicals are not typically filtered out before being dumped back into the ocean.
But let's just assume they filtered out all those chemicals and only dumped saltier concentrated sea water back into the ocean. The area around the dump site would still be a death pit for any and all marine life very close by. Now you can get away with a few of these plants, sure. But if you start trying to provide enough water for, say, the entire state of California, through desalination alone, you're going to absolutely murder the coastal ecology.
Again, I'm not an eco-freak. I think we should have maybe a dozen or so such plants (maybe a few more) to help offset the drought and tough beans for some of the sea life involved. But if you scale it out too much you will wreck the coast.