Water comes to symbolize many concepts in the novel. One example is in Antonio's last name, and one of the contrasting aspects of his personality. "Marez" implies a connection to the sea, expressing the wild, untamed, almost destructive nature of water. Of course, this capacity for destruction is also captured in the myth of the golden carp. Yet the carp offers an alternative to the rather oppressive presentation of Catholicism in the novel. Thus, water also symbolizes a freedom or escape from that religious control.
As the previous poster mentioned, there is a clear division between the symbolism of the river and the pond. This division is connected to the contrast between the society of the town (and its influence on Tony) and the hidden secrets of the land surrounding it. These are also important influences on Tony's development, with the violence and debauchery of the town infecting the river, while the pond carries the sacred purity of the llano, untouched by human sin. There is also a suggestion of baptism in the novel, particularly when Tony witnesses Lupito's death in the river. It's his baptism into the dark side of human nature. Florence's death by drowning could be seen as a reverse of this, perhaps connecting to his own questioning of the tenements of Catholicism.