Well first you need to let it dry for days... with the battery out if at all possible. Water does way loads of damage to devices but if the battery is in and wet that is worse. Also remove any SIM or SD cards.
The more water and the longer it was in it, the longer you need to leave it turned off and drying. We are talking days, not hours. Longer is way better.
Once dry, for sure, then try turning on. It may stay on long enough for you to get data off of it via a USB cord connected to a computer.
However, if it won't stay on at all, there really is no way to get data off of it.