Jehoshaphat uses a rhetorical question here to state that he is certain that there is a prophet there and to find out where he is. This may be written as a statement. Alternate translation: "I am sure there is a prophet of Yahweh here! Tell me where one is, so we may consult Yahweh by him." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This idiom means that he was Elijah's helper. The phrase "poured water on the hands" is a description of one of the ways he served Elijah. Alternate translation: "who was a helper to Elijah" (See: [[rc://en/ta/man/translate/figs-idiom]])