That's the major issue with desalination. Dumping the brine byproduct back into the ocean is roughly equivalent of dumping other chemicals into the ocean, if not worse.
Are you claiming this is comparable to an oil spill or eutherification?
Now, it can still be just as toxic, and that has to be handled. But if properly diluted, then it should be equivalent to the ocean evaporating water, which happens constantly.
That's the key phrase here. Obviously, if you dump it back into the ocean such that it actually does not impact local salinity levels, its fine. But that's also true for essentially anything dumped into the ocean - if you can manage the dumping of a toxic chemical such that it has no impact, wouldn't it also be fine?
Of course if you go down that route much you realize that the output of sewage plants is safe for human consumption there is no technical reason not to put it back into the water supply and greatly reduce the need for makeup water.