Water is definitely a symbol for life in this book, rushing along and changing shape much as the characters' lives do.
How do we know this is water's function in the novel? Well, the characters themselves make the connection. For example, when Marie is reflecting on how she and Rushes Bear finally managed to gain some mutual respect and understanding, she uses water metaphors to describe the similarities in their experiences. They finally bonded, in her view:
… [b]ecause we shared the loneliness that was one shape. Because I knew that in her old age she shared that same boat, where I had labored. She crested and sank in dark waves. Those waves were taking her onward, through night, through day, the water beating and slashing across her unknown path. She struggled to continue. She was traveling hard, and death was her light. (5.2.50)
As you can see there, Marie suggests that water and life both bounce you around, make you change direction, drive you on (whether you want to be driven or not), and disorient you. And Marie and Rushes Bear were, in Marie's view, in the same boat on that journey.
Nector, too, makes water a centerpiece in his musings on life. When he realizes that life has basically just pushed him along relentlessly without his really noticing, he likens that forward movement to a current… and the moment in which he actually slows down enough to notice to a pooling in the water:
And then it was like the river pooled.
Maybe I took my eyes off the current too quick, Maybe the fast movement of time had made me dizzy. I was shocked. I remember the day it happened. (7.1.33-34)
After this little pause in Nector's journey down the stream of life, he finds himself pursuing an affair with Lulu in order to "swim against the movement of time" (7.1.39)… and the water metaphors carry right through the end of the affair.
When Nector decides that he's got to let Lulu go, he takes a literal swim as he tries to straighten out the metaphorical "currents" of his love life:
I swam until I felt a clean tug in my soul to go home and forget about Lulu. I told myself I had seen her for the last time that night. I gave her up and dived down to the bottom of the lake where it was cold, dark, still, like the pit bottom of a grave. Perhaps I should have stayed there and never fought. Perhaps I should have taken a breath. But I didn't. The water bounced me up. I had to get back in the thick of my life. (7.1.127).
Of course, the next day, he decided that he was going to propose to Lulu…
But anyway, you get the point: Nector and Marie both liken life to the movements of water, which can push, pull, and overwhelm us. Another prime example of this symbol's use is Henry Junior, of course, since he escapes from his own ocean of troubles by drowning himself.
Also, we'd be falling down on the job if we didn't mention that water becomes important in the references to June as a kind of Jesus figure who must journey (or walk) across the (frozen) water to get home.
That first journey—at the beginning of the book, when she's tromping through the snow—is fatal, and she doesn't make it home. However, at the end of the book, her son Lipsha kind of metaphorically brings her home across the river (in the section named "Crossing the Water"), and the moment kind of reads like been brought back to life (or resurrected) in that moment. So, in these examples too, navigating water is like navigating life… and vice versa.