What does Winnie do with the bottle of spring water Jesse gave her in Tuck Everlasting?

Winnie pours the spring water onto the toad.

Winnie accidentally sees Jesse drinking from the spring in the woods near her house and ends up going on a grand adventure. She learns that he is immortal, and so is his entire family.  Jesse is seventeen, but he is really one hundred and four years old.  He will be seventeen forever.

Jesse is quite smitten with Winnie, even though she is not old enough yet to be a girlfriend.  He asks her to wait until she is his age and then drink the water and become immortal too.

But the thing is, you knowing about the water already, and living right next to it so's you could go there any time, well, listen, 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! (Ch. 14)

Winnie is “struck dumb” by this offer.  Winnie also likes Jessie quite a bit.  She is too young to make a decision about who she wants to spend the rest of her life with, let alone eternity.  Jesse tells her to think about it.

While helping Mae Tuck escape from prison, Jesse gives Winnie a bottle of the spring water and reminds her of her options.  Later, Winnie sees a toad she has befriended being harassed by a dog.  She decides to get the bottle Jesse gave her and use it to help the toad.

The toad still squatted where she had dropped it, the dog still waited at the fence. 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. (Ch. 25) 

The decision is not necessarily as meaningful as you might think.  At the time, Winnie felt that she could get more water any time.  A fire in the woods eventually eliminates that possibility, but Winnie has made her choice.  She wants to be mortal.  She grows up and dies, and leads a normal life.  Her experience with the Tucks seems to show her that being immortal is not all it’s cracked up to be.  Dumping out the water that Jesse gave her was a hint at her future choice not to be immortal.