30 hp on a 20 ft pontoon..........it will be slow and like a barge, so go with the lowest pitch for the most thrust and highest rpms....you definitely want a 4-blade...think of it like being an outboard on a sailboat
Solas Amita 4 or even Solas Amita 4 High Thrust (if they make one for the engine you have), has the hub already pressed in and ready to go, most economical option there is and easiest to install
FYI - Wholesale Marine has a sale going right now through 11/25, 15% off and free shipping on orders over $50, code = PRESALE