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It depends if they boil the sea water down to nothing. If they only boil away a fraction of it, the salt is still present in solution in a concentrated form. It can be sent back to the ocean with little effect as long as you spread it over a wide enough area.

If reverse osmosis is used, it never desalinates all the water, just a small amount of it. The leftover water has a slightly higher concentration of salt which again can be returned to the ocean as long as you spread it out over a wide enough area to prevent a local concentration of salt.