The religious element of the book begins with santiago reciting the catholig "hail mary prayer" he adds a prayer that mary kill the fish. why do you think this is unusual?

We usually don't attribute prayer to killing something. The Marlin isn't just food, it is all wrapped up with Santiago's identity, pride and masculinity. In that context the prayer may be seen as unusual.

Throughout the book it never mentions Santiago going to church or his religious belief. He is a poor old man on the African seaside, no one would expect him to be Catholic. Santiago refers to the Marlin as his brother. A hail mary prayer is said to repel and forgive sin, killing your brother is a sin.