Then run the exhaust pipe further out to sea, and/or mix it in with some ocean water before discharging. That's just an engineering problem. Nothing is free. If we harvest rainwater, that's depriving plants and animals on the continent of water they might have used. If we harvest groundwater, it isn't sustainable, it can cause sinking, and it takes from plants that might have been able to tap that water. I'd argue that desalination is among the least impactful sources of water. The world ocean has far more water than we could dream of using on the continents. It's the only way anyhow. Groundwater is unsustainable and relying on rainwater is dangerous at best, see Brazil. Finding ways to build the intakes and outputs so that their impact on nature is minimized is an engineering problem, not a showstopper.