This novel by Louise Erdrich is a saga of two Native American families and several decades in their lives, mostly narrated by various family members, each of whom gives voice to a separate chapter in the book.
The element of water figures heavily in this story where the characters live very close to nature. In literature, the element of water is often symbolic of various emotional states or transitions. Water can be associated with romance or sex, or symbolic rebirth and renewal.
In the chapter on Lulu Nanapush (entitled "The Island"), the journey by boat establishes water as a path to love, a place of transition to a new life. After Lulu manages to break down Moses Pillager's emotional walls, and the two become lovers, the water is a constant presence and symbol of renewal in their (previously lonely) lives: they bathe in the cold water of the lake with steam rising off their bodies. In this way the lake water is a symbol of change and alchemical power: they transform the icy water to steam with their bodies, and it suggests the passion and heat of their love is powerful indeed.
Water's characteristics are that it is fluid, ever changing its shape to fit the path it is given, and it is a source of life. As carbon based organisms, we must have water to live. This story is a story of family and, as such, water is a suitable symbolic image for that theme. Family is our source of life, and it remains with us and as a part of us throughout our life. Family is also fluid, changing because of births and deaths, and conforming to the relationships of its members.