Chances are the ECM is toast internally , but you could try to open the back cover and use some good non-residue contact cleaner like CRC or equal and let it dry with some air or air blasts. Clean the three plug female connectors also and give it a try again.
If you need a new ECM, there are no cheap ones , when on the used market they sell for about $500 to $1000 and new ones go for about $1500. Obviously very expensive when boat parts that are not supposed to go swimming make a splash!
Ray @ Raylar