Face ID not working after iPhone fell in water

Hello,

If you have a case on your phone, remove it and let everything dry out for a few days. Also see this:

About splash, water, and dust resistance of iPhone 7 and later 

Water and other liquid damage to iPhone or iPod isn't covered by warranty 

Good luck.

Water intrusion is often fatal and not covered under iPhone warranty if your phone is still covered under warranty. Let it dry (don't put it in rice as that's an old wife's tail, which is ineffective), but be prepared to either purchase an Out of Warranty replacement phone or a new phone depending on what you'd rather do.