Rinse in fresh water as soon as you can, wash with warm soapy water, rinse again, then dry completely and oil the blade. Salt does not evaporate with the water. It stays behind as crystals that pull moisture out of the air and keep attacking the steel long after the blade looks dry.
Chloride from salt is specifically good at breaking down the passive chromium oxide layer that makes stainless steel stainless. Left on the blade it keeps working, which is why a knife that looked fine on Sunday is spotted by Wednesday.
Fresh water is the whole answer. Rinse first, wash second.
Fresh-water rinse, warm soapy wash, fresh-water rinse again, then dry thoroughly, including inside the pivot on a folder. Work the blade to move water out, then leave it open to air dry before oiling.
Finish with a light oil on the blade and a drop at the pivot.
Never store a wet knife in a leather sheath. Leather holds moisture against the steel and is the single most common way good blades get pitted.