It was a double whammy of her breaking one of the back camera lenses which led to the water ingress when she then later dropped it into a tub of water. She didn't tell me about the broken lense as I could have probably dealt with it on insurance before the water ingress. Teenagers and phones...
Make sure nothing is covering the front cameras like a screen protector etc.
Settings - Face ID & Passcode, Reset Face ID then try again.
My daughters XR suffered water damage and this stopped the facial recognition from working, even when I reset the face ID as soon as I went to set it up it...