I would count Fredo's Hail Mary's as more of a good luck gesture, akin to carrying a rabbit's foot or wearing your lucky fishing hat.
Yes, Fredo's Hail Mary had brought him fishing luck in the past. But, there is another GFII script where Mary in Heaven says "I was hoping he was praying not to get shot, but, alas, it was only to catch fish. My Son is better at this than I am."