Not all seawater is the same…
Most seawater has a salinity of around 31-38 mg/l.
Some seas with restricted inflow and high evaporation levels (such as the Red Sea and Persian Gulf) have much higher salinity. Some lakes with restricted outflows (like the Dead Sea) even reach saturation levels, resulting in salt precipitating out onto the sea floor.
Conversely, where fresh water mixes with sea water, salinity levels can be much lower. Desalination of low salinity water is much easier than of high water, this is why estuarine locations may be preferred for desalination plants.
There was a time, not long ago, when the most we did to deal with wastewater was pipe it into the sea. In many places the highly saline brine from desalination plants appears to get the same treatment. Just as not all land is equal in biodiversity or ecological sensitivity, different areas of the sea may require different levels of protection. Detailed bathymetric and hydrological studies are required in order to minimise the environmental damage that may be caused by marine discharge.
While it is easy to say that all waste water should be treated completely before discharge, the energy & chemicals required for this have their own environmental impact, so a balance must be achieved.