It won't kill anything. The surface of the ocean has natural fluctuations of salinity all the time due to evaporation, and it doesn't cause mass die offs. It's literally a necessary part of the ecosystem (and in fact discharging freshwater can cause problems to the ecosystem). But if your discharged water is within the natural 30 to 40 grams of salt per liter range, there's no ecological problem here.
It'll kill however, plenty of things.