There can be an environmental hazard when waste salt is dumped resulting in toxic levels of salinity locally but overall increase in ocean salinity is negligible.

On the flipside, desalination might very well pay for itself in the future when paired with operations like the harvesting of lithium, hydrogen and chlorine from ocean water. https://www.intelligentliving.co/desalination-device-harvests-lithium-h2-from-seawater/amp/