Are you talking about the brine? This is trivial if you just put a pipe a few hundred yards out to sea. The concentration is diluted extremely quickly. (The only exception is if your desalination plant is below sea level because then you have to pump the brine up hill, which costs some energy.)
I've seen the assumption that the brine dissipates quickly challenged.
Fluids can remain subbornly un-mixed, over large distances and for long times. Given that brine is heavier than seawater, it will almost certainly tend to sink and flow along bathymetric contours, perhaps pooling in local low spots. And sealife can be exceedingly sensitive to changes in temperature, ion density, salt content, etc.
This reminds me of a book in which an ecologist described a discussion with a chemist over the concentration of some pollutant in seawater. Roughly, "Assuming a well-mixed solution..." starts the chemist. "How are you planning to stir the oceans?" asks the ecologist....
