Symbolism, Imagery, Allegory
The image of water in Sister Carrie is almost always a signal of trouble, usually of the financial sort. Because water has the ability to completely overpower humans (think of storms, shipwrecks), it is the perfect image for suggesting that economic forces have a similar ability to overwhelm and overpower humans and their intentions (a huge part of what the novel is all about, as we see through Hurstwood's character).
It's no coincidence then, that water imagery pops up in moments of financial crisis or impending financial crisis. Take, for instance, depictions of Carrie's grueling effort to find work in the novel's beginning: "She felt the flow of the tide of effort and interest—felt her own helplessness without quite realizing the wisp on the tide that she was" (3.76).
Consider as well:
Whatever a man like Hurstwood could be in Chicago, it is very evident that he would be but an inconspicuous drop in an ocean like New York… The sea was already full of whales. A common fish must needs disappear wholly from view—remain unseen. In other words, Hurstwood was nothing. (30.1)
Water doesn't just reinforce personal trouble—it's also used to express the plight of whole classes of people in the novel. As the narrator notes of the homeless New Yorkers:
They were all pale, flabby, sunken-eyed, hollow-chested, with eyes that glinted and shone and lips that were a sickly red by contrast… They were of the class which simply floats and drifts, every wave of people washing up one, as breakers do driftwood upon a stormy shore. (47.2)
Rather than the nice, lovely image of water as a life-sustaining entity we sometimes encounter in literature, water in Sister Carrie most of the time conjures up dark associations of drowning and death.
So whenever you encounter water in this book, be sure to think of the scary Jaws music.