Jesse Tuck gives Winnie Foster the bottle of spring water in chapter fourteen. Jesse stumbles over his words in an effort to explain why he is giving her the spring water, but he eventually makes it clear that he would like Winnie to drink the water when she is seventeen like him.
". . . how'd it be if you was to wait till you're seventeen, same age as me—heck, that's only six years off—and then you could go and drink some, and then you could go away with me! We could get married, even. That'd be pretty good, wouldn't it! We could have a grand old time, go all around the world, see everything."
For awhile, Winnie seriously considers Jesse's quasi-proposal, but she never ends up drinking the spring water. Instead, she uses the spring water to give eternal life to the book's reappearing toad. What prompts her to do this is the fact that in chapter 25 the toad is being pestered by a fairly aggressive dog. Winnie pours the spring water over the toad so it never has to worry about being attacked by that dog, or any dog, ever again.
Winnie pulled out the cork from the mouth of the bottle, and kneeling, she poured the precious water, very slowly and carefully, over the toad. . . Then she stooped and put her hand through the fence and set the toad free. "There!" she said. "You're safe. Forever."