"Everyone who drinks of this water will thirst again; but whoever drinks of the water that I will give him shall never thirst; but the water that I will give him will become in him a well of water springing up to eternal life."
In this passage, Jesus uses water as a metaphor for salvation, with special emphasis on the water of salvation being a gift, and even more emphasis that one drink is all that is necessary for everlasting life. Drinking water from another source will not ultimately satisfy your thirst… you will become thirsty once again. But whoever drinks of the water that Jesus provides will never again become thirsty, because this 'living water' is eternal life.
And once again, this only makes sense… because if you had to keep drinking over and over again in order to keep your 'eternal life', then you couldn't very well call that life "eternal."