douse, dowse

Douse or douse means to soak with water or liquid; to throw water on--
He used the dishwater to douse the fire, and then stomped on it to extinguish it completely.
If the dog starts barking at nothing again, just douse him with a cup of water.

Dowse or dowse means to divine the location of water underground, by means of a forked stick, using special knowledge or magic--
He learned to dowse from his grandfather, who showed him how to look for water underground.
When they heard he could dowse, they hired him to tell them where to dig the well.

Now that you know that, you can say--
"He offered to dowse for them, and said that they could douse him with water from an underground spring  that very day."