Can WD40 be used to mitigate water damage on electronics?
The standard method I have used is to wash in distilled or de-ionized water. Then put into a bath of iso-propyl alcohol, often by submerging the board for some time. Air dry for a while to get rid of most of the IPA, and then put into a temperature controlled oven at 60degC for a few hours.

Unfortunately, if batteries were connected and the device powered when it was submerged it may have caused damage especially if it was saline. You will just have to try.