Face ID not working after getting into water

That kind of issue cannot be diagnosed remotely, sorry.

I suggest that you make an appointment and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector.

After checking it out, they'll give you all the available options.

Regards

Giulio

Suggest begin backing-up your iPhone VERY regularly.

Any water damage can cause other delayed issues or failure.

Your iPhone is simply no longer as reliable as it was previously.