pontoon boat prop question

[oteps]

I need a prop for a 24 ft pontoon with a 90 hp 4stroke mercury. The # on the prop is M?8 77338A45 11P. Im guessing its an 11 pitch? THe boat is used on the Susquehanna river, pull a tube with 1 person on it a lot. I think the boat could use some more top speed. What pitch should I be looking for? Is a stainless worth the extra money, it has an aluminum on it now? There is a lot of rocks to hit in the shallow parts.