Hold (insert button here) To Drink Water?

ive seen a lot of games where you can walk up to terrain water and press e on it to run a script and others where you can cast a ray from the camera and detect if a players camera is underwater but i was never sure how to script with terrain water like that the only thing i could ever find is how to generate terrain water but i dont exactly need that and wanted to know if anyone knew how to do this? mostly the first part i can figure out the camera underwater myself