My own personal recommendation would be that you go for the golden carp rather than water, as it is a much bigger and more important symbol with plenty of quotations that you can pick out to support your argument. The way in which the golden carp symbolically represents a rival religious belief separate from Catholicism is vital to Antonio's rising conflict between the received religious faith of his mother and his understanding and acceptance of a different way of understanding life and the divine, and is linked to the way in which the novel suggests every religion offers equally valid but different approaches to living and understanding who we are the world we live in. Antonio's acceptance of the golden carp later on in the novel represents his own understanding of how we are able to turn to all available sources in our efforts to establish our own sense of identity and finding our own place in life, even if these sources at first appear to be contradictory. Note how the golden carp is first presented:
The orange of the golden carp appeared at the edge of the pond... We watched in silence at the beauty and grandeur of the great fish. Out of the corners of my eyes I saw Cico hold his hand to his breast as the golden carp glided by. Then with a switch of his powerful tail the golden carp disappeared into the shadowy water under the thicket.
This quotation comes from Chapter 11 and is Antonio's first sighting of the golden carp. The "beauty and grandeur" of the fish stresses its divine nature, as does Cico's act of worship by holding his hand to his breast. This confrontation is significant because it represents Antonio's first meeting with a rival belief and religion and leads to his final realisation in Chapter 22 that he is not forced to choose between the rival conflicts that threaten to rip him apart, but that he can "Take the llano and the river valley, the moon and the sea, God and the golden carp--and make something new."