Face ID stopped working after falling into water

Hello ~ You really answered your own question. Your device has water damage. Take it in to Apple for a checkup:

iPhone Repair and Service - Apple Support

~Katana-San~

Click on the Get Support link at the top right of this page and start the service process.

[Edited by Moderator]

An iPhone cannot be submerged. Water and electronics don't mix. You may have to replace your device.