Waterbenders deal with the flow of energy; they let their defense become their offense, turning their opponents' own forces against them. The technique of redirecting Lightning, which is a Firebending attack, is based off of this fundamental Waterbending concept.
Guy#2: Yeah, I think it's Guy#3, I saw him drinking out of the pitcher this morning.
Guy#3: Sup? Who's been bending the water today?