That's not how desalination works in practice, you don't end up with 100% salt as a result, even so that would be problematic. Typically you end up with about h

That's not how desalination works in practice, you don't end up with 100% salt as a result, even so that would be problematic. Typically you end up with about half of the inflow salt water being converted to fresh water and the other half being converted to higher salinity salt water. Note that the salt in seawater is actually about 3.5%. So what happens is you suck up a bunch of seawater, which is pretty easy to mitigate negative effects from, and then you have an outflow of 7% salinity seawater, which you have to be somewhat careful with. Ignoring power usage, the US draws something like ten thousand cubic meters per second of water for its use. So a single plant that is supplying water for, say, only a million people might be expected to discharge a volume of water brine that was on the order of 1 million cubic meters a day. The ocean is big, but it would be extremely costly to discharge that amount of brine into the ocean in a way that wouldn't create a very large persistently briny volume near the discharge point. Also, that discharge point is almost certainly going to be near to the shore, which means that it will impact ocean life.