What to do if my iPhone got wet and won't charge?

According to Apple, if your iPhone has been exposed to liquid, unplug all cables and do not charge your device until it’s completely dry. Allow at least 5 hours before charging or connecting a cable. Face your iPhone and charging port down so that it can dry.

If after that your iPhone wont turn on, I recommend you take your iPhone to your nearest Apple Store to have it taken a look at, though you're probably looking at a new iPhone.

⇢ Genius Bar Reservation and Apple Support Options - Apple ⚛️

Unfortunately, salt water is the enemy of any electronic device.

Sorry, but your phone has been permanently damaged and repairs are not possible, so you will be looking at a replacement phone.

Sorry but your phone is toast, especially with salt water. There is nothing the Genius Bar can do at this point. Time for a new phone.