Water Damage.

[stroths]

The best thing to do is take the battery out immediately. If its salt water you want to dunk the phone in clean water to remove the salt as it will cause issues down the road.

Put it in a plastic ziplock with as many of those silica packs you can fit in there to absorb the moisture. You can get these from your local pharmacy as they have tons of them from the drugs.

Put the sealed bag under a desk lamp to let it heat up a little and speed up the moisture extraction.