Most desalinization processes don't produce "salt", but extract a certain amount of unsalted water from the in put, leaving behind more salty water. Typically reverse osmosis at best reaches 1:1, that means you need 2 units of input water and get one unit of desalinated water and one unit of salty water. For sea water desalination 1:1 is quite optimistic though. This salty water can be safely put back into the sea. With very large scale plants you might want to make sure that you don't release the salty water at a single very concentrated spot.
These 1:1 are the best numbers for demineralizing sweet water - I assumed that salt water desalination had much different proportions, which also means, that the relative change in salt content is pretty small and consequently the impact of putting it back into the sea.
