Try thinking of this another way to help you understand. How does nature get most of us surface freshwater to begin with? Energy from the sun heats ocean water which causes evaporation into the atmosphere (aka "natural desalinization") and then wind patterns in the atmosphere (also generated by energy from the sun) cause clouds to move over land where water vapor eventually condenses and falls as precipitation, filling lakes and rivers.

So the equation is basically energy input from the sun creates freshwater from salt water and transports it over land. Humans can, and have, replicated this process artificially, the limiting factor just being the cost of energy.

You also need seawater. But infinite energy lets you move fresh water a long distance. You could deliver it to the middle of the country from the coast. Or from other countries.

Yes, but that's localized and temporary. It's nothing like the ecological impact of burning fossil fuels, and more like the ecological impact of a quarry. It's a lower impact than a hydro dam, I think.

You have an intake that can suck in sea life accidentally, and an outlet that emits water too salty for sea life to survive, which doesn't go beyond a certain distance as it mixes again with normal ocean water.