I am trying to learn all I can here..... so please forgive my ignorance.

Let's say you have a boat 29' (for arguments sake) and it has a very nicely built 502, but no blower. Let's say I buy a procharger and install on said boat.... I know I would need to re-prop it, but do I have to change the gears in the drive? If so, is this an easy swap? Or does it require a completely different drive setup?

no gear change needed....just a re-prop

Again showing my ignorance.... on the procharger website, they show the drive gears as being different. Is this just to maximize the gains, or is this because that is what gear they had in the drive when they did the testing?

that's true assuming the drive is strong enough to handle the new HP....still wouldn't require a gear ratio change, just a stronger drive if the HP gets too high.