No...Most ocean life is ridiculously dependent on perfect salinity levels. We wouldn't be doing any favors by increasing these levels, no matter how small our impact may be.
The change in salinity would obviously be negligible averaged over the ocean, and would be massively, massively dwarfed by other human factors. Your extreme use of the precautionary principle would prohibit essentially all new constructions, if applied widely.
Much more interesting is the question of the impact to the nearby ecology in the immediate neighborhood of the discharge pipe. I'm not familiar with this, but I'd wager it can easily be mitigated.
EDIT: Thank you to CHY872, who provides a link below to a review of the environmental impact of the waste water from desalinization:
> In most other cases environmental effects appear to be limited to within 10s of meters of outfalls.