There's only 1 prop for your boat, and that's the one that allows you to run your engine at the munufacturers recommended max rpm at WOT. It's matched to the gear ratio in your outdrive. Manufacturers equip the boat with the correct prop/gearing when they build it. They do take several assumptions as to how much fuel, people, gear, and temperature/humidity, driveability at all speeds and make the best compromise. Maybe your useage (loading) is far different than the average that the manufacturer used, and you could go up(or down) a bit in pitch. Does you boat run at max rpm at WOT?
Yeah, you can change pitch to give you extra speed, but your also gonna pull more rpm, and increase the torque loading on your engine when you get up on plane--both will reduce the life of your engine. Think of your boat as having only 1 gear, and that gear is determined such it allows you engine to turn max rpm at WOT. What you're looking for is a multiple gear transmission. Both Mercruiser and Volvo are working on them (2 speed trans), but they're not in production.
