Answer:
The author did this to reinforce Florence's lack of faith and religiosity.
Step-by-step explanation:
Although you have not presented the book this question refers to, we can assume, through the context of the question above, that you are referring to the book "Bless Me, Ultima" which tells the story of how Antonio Marez, a Mexican boy, raised in an extremely Catholic home, struggles to discover his identity and religiosity, even in the midst of a strong family influence.
Within this story, when Antonio and Cico find a golden carp and decide to show it to Florence, they aim to show Florence a god who is not vindictive and punitive, as they see the golden carp as a deity. Meanwhile Florence drowns before Antonio and Cico tell him about the golden carp. The author decides to do this, to highlight Florence's lack of faith and religion, showing that the golden carp would make no difference to him.