PIR isn't a good idea as they detect moving temperature differences with "zones".
Moving fingers while washing hands won't be enough most likely.
Same for radar detectors.
Ultrasonic sounds like a good idea to me.
LisaM had a modified plugin for sensors beside HC-SR04, some water resistent versions might exist...
I don't know if any of the code made it into ESPEasy Mega.
Another idea might be an active infrared sensor instead of the passive PIR.
A well known manufacturer is "Grohe" in Germany.

- 36236001.jpg (6.03 KiB) Viewed 11405 times
The infrared light barrier (Sender & receiver) resides inside the tap's socket behind the dark oval.
Should be possible to make something similiar.