It's called Faraday's Laws of Electrolysis. Salt water is the epitome of what you need to set up the chemical reaction that, in this case, destroys the components and the board itself.

The RFID chip that the skreem reads to start the car is sealed and powered by the ring around the key - so the car should start fine. BUT, the small circuit board that holds the transmitter (which is battery powered) is not sealed at all, it is this assembly that the lock/unlock/panic buttons are using to signal the car.

You need a new key. Contact Rob at Needswings.com