Ultima's owl represents comfort. For Antonio, it is a reminder that she's not only to protect him, but as a source of protection as well. \
The carp is often a source of guilt, as Antonio sees his interest in the magical carp as turning his back on his religion. Thus, the magical carp is not initially embraced.
I've never seen the river as symbolic. It is a force.... and it becomes a way to commune with nature. Antonio hears the voice of the river in Chapter Two, and it serves to discomfit him.... like it's warning him that conflicts are approaching his otherwise peaceful existence.