Evaporated sea water will have a certain amount of calcium chloride in it which is hygroscopic. It will absorb and retain moisture from the air and in humid conditions it will actually dissolve and form very corrosive little pools. You should take it in to an Apple store and have them vacuum or blow out all of the salt granules that may have gotten into and under the keys and the rest of the interior. If it was mine, I'd get some silica gel, put it into a cloth bag and put that bag and the computer into a plastic bag and keep it there until I could get it in for servicing. Apple probably won't do the cleanup for free but I'd only trust them to do the job if it can be done at all. Good luck. Rick