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.
Edit to the last answer, I said Lyman found his father when I meant Lipsha.
It is complex and contradictory throughout the novel. Water is violent throughout the novel, the antithesis of water as a life giving element to the Native Americans. The dysfunction of this community is seen in the first chapter with the near drowning of Lynette (in the kitchen sink), with the drowning of Gordie (not literally) in the last sentence of "Crown of Thorns," with the Red Convertible and Henry's death ("My boots are filling"), and June's "walk[ing] over it like water and came home" on the 6th page. Here water is perverted and distructive--unlike the archetypal water symbol. At the same time, hope is restored in the last chapter (on the last page), as June's son has "nothing to do but cross the water, and bring her home." Such hope for the community on the last page, hope beyond Lyman's future casino and beyond turmoil and alcoholism of the novel. Lyman has found his father, found himself, and is ready to re-establish his place in a culture marginalized by religion and "white" society. Continue studying water--bridges, plunges, water rolling over "the beads," etc. It is a complex and worthy motif to explore in her work.
The previous answer is correct in a basic literary sense, however Erdrich utilizes water in other ways as well. Water is the element of East, sunrise, yellow, spring, and begining in Native American society. Water in dreams can represent one's knowledge of the opposite sex. If one looks through Erdirch's stories with this basic knowledge of this element the even deeper meanings will be revealed.
The source below will explain these elements better, but the information stated is from a lifetime of tribal reservation education.