Water is one of the five great elements, water is the everchanging element, which comes in 3 general forms, steam/air, water/liquid, and ice/snow. It is a hard element to master, because water jutsus may be powerful, but they can be terrible to a master who uses them without thinking. Water is strong against fire, one of the other five great elements, water can put out the fire, simply by cooling it down, because of this, water (obviously) is a good weapon against a fire user. Water sadly also have a weakness against the force of lightning, water lets the lightning travel throught it, so a water user must watch out for this, but the lightning can also be a clever ally, if used correctly lightning and water can combine into terrible jutsus, that'll send shock-waves at ones opponent. (shock-waves you get it xd)