Face ID not working after a dunking

Of course, it is totally up to you on what you want to do in this case. However, I would strongly suggest that you do not power-up your phone, and make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look and provide you with your options.

The issue is, even if all of the liquid evaporates, it could leave behind conductive residue that could potentially cause more damage to the phone. Since FaceID is not working can indicate that liquid has, most likely, already entered into the phone.

You can wait til it has completely dried up and try FaceID again.

If you can't wait, then best to go to the nearest Apple Store or Apple-authorized repair center.