The bottle of spring water has many symbolic meanings for Winnie. To better conceptualize these meanings, consider another symbol that is referenced throughout the book, including the first sentence, the wheel. Life is often considered an unending cycle, much like a wheel. As the wheel turns, so does each individual's life; with the years adding on until death. Each new life can only be completed by death, unlike for the Tucks. They are no longer on that wheel, and are unchanging just as the rowboat. The bottle of spring water symbolized the differences in life and change. The water is a living symbol of growth, vitality, and for those who drink it, gave them immortality. Unfortunately then, the water also represents temptation.
Tuck Everlasting presents a religious allegory with the bottle of spring water.
Remember that an allegory is a story that has a surface level meaning, and a deeper, hidden meaning.
The bottle of spring water signifies the same temptation that Eve felt in the Garden of Eden, for Winnie now. She is deeply tempted to be free, and drink the water. However, doing so will mean that she will forever be the same age, watching as her family dies and moves on without her. Winnie must struggle with her curiosity versus her judgment.
Lastly, the water represents hope and choice. Remember that Winnie has to decide what is most important for her, freedom forever, or life and its turbulence. She, in turn, gives the water to her toad friend who she believes will have an easier time living forever.