Given the ending of this novel, it’s not really surprising that water and floods are an important symbol. In fact, water and flood imagery is found throughout the novel, and the river itself is practically a character. We often hear about water in direct relation to the Tulliver kids, which is sort of morbidly appropriate given the way they die. Mrs. Tulliver is always complaining that Maggie is going to fall in the river and drown one day.
But water isn’t just a bringer of death and doom and destruction and other "d" words. The river is often a place of romance, dreams, and even magic, particularly for Maggie and Stephen:
They glided rapidly along, to Stephen’s rowing, helped by the backward-flowing tide [...] on between the silent, sunny fields and pastures [...] thought did not belong to that enchanted haze in which they were enveloped - it belonged to the past and the future that lay outside the haze. (6.13.33)
But the river always carries a darker edge with it. Maggie is lulled by the river and the sleepy, romantic atmosphere it produces. But the river helps to carry Maggie into a nightmarish situation, where she is tempted to do something that she worries is morally wrong.
As a symbol, water and the river contains both darkness and light. Floods, too, are positive and negative in this book. While floods can be destructive forces, they are also somehow cleansing, in a highly biblical sense:
Nature repairs her ravages – repairs them with her sunshine and with human labour. The desolation wrought by that flood, had left little visible trace on the face of the earth, five years after. (7.6.1)
This blending of good and bad elements is important to the other major role water symbolism plays in this book. Water is a metaphor for narrative itself. Here’s how the narrator describes Maggie’s story using water as a metaphor:
Maggie’s destiny, then, is at present hidden, and we must wait for it to reveal itself like the course of an unmapped river: we only know that the river is full and rapid, and that for all rivers there is the same final home. (6.6.4)
Water represents individual stories and life itself here, which is of course filled with both triumph and suffering.