As a general rule... when it comes to power adapters, it's best to just get a new one. Anytime you're working with something where you're input is live current from the wall, you really shouldn't take a chance. While the internal circuitry should prevent any damage to your MacBook... it is always difficult to determine what damage may have been caused to individual components. If something was damaged by the water it can cause it to simply not work at all... or some of the internal safety components could have been damaged which can cause anything from poor voltage regulation, to overheating, to possibly shock to you or your MacBook. While adapters are usually pretty rugged, it's generally better not to take the chance.