Take the battery out immediately. Put the battery and the phone in a ziploc baggie of rice
A few years ago, we were able to let that one dry out without any problems. Last year he did this w/ a new phone, and no go--let it dry out for about 2 weeks w/ no luck.We tried the rice trick when we washed my son's Itouch a couple of weeks ago. We put it in a ziploc bag of dry rice, laid it on a table under a tall lamp, and left it for a week. When we took it out and tried to turn it on, nothing. We just figured it was toast. Then someone suggested that maybe it just needed charging. Duh. We put it on the charger and it came right up!
We are now true believers in dry rice!Good luck!

rice absorbs water... why do you think people put rice in salt shakers?