Joe is absolutely correct on bigger motors get better gas mileage--IF, repeat, IF--the boat is re-propped to use that increased power. If not, you end up with a drag boat that will burn more gas than before. (DItto for cars and trucks without a nice, high geared ovedrive the newer, stronger
motor will pull more often.)
PS: As a long time hot rodder (60 years plus) I just LOVE to improve the power of an engine. Love it! That said, an engine designed for a cruiser needs more torque and less hp. If not, you'll burn more gas (without re-proping) and send the unwary FLYING when some squid rams the throttles open. Still....
Engines, Shafts, Steering, Struts, Rudders, etc.