Does diluting it with seawater (after which I assume it is returned to the sea?) result in an endlessly increasing salt level in the seawater, over a long period of time, that will reach a dangerous level at some point?

Edit: I realize my naivety now. Other comments mentioned the desalinated water (at least some of it) will end up back in the sea eventually, so that would maybe counterbalance such a concern.

The short term effects are increased salinity and reduced oxygenation for water in the immediate vicinity of the desalination plant, which is enough to drastically alter the biome (aka “kill all the sea life”).