Water is very important to all life. While people can survive 2 weeks without food they could not stand 2 weeks without water. Water can be used in defense against jutsu based on fire(you could put out the fire), Lightning(water can conduct lightning), water (they would cancel out) and wind(the water could turn to ice and stop the wind)and Earth(you could use so much water the earth turns to mud).Also with attack you could easily decapitate somebody with concentrated water, combine water with the lightning element and you could also drown somebody from nonstop amounts water.