Buy a boat with a bigger engine. Or better yet... 2 engines.
OK... I know that sounds like a jack-off comment... but that TINY LITTLE engine already makes a TON of power. It's under 800cc's, and makes 110 hp.
Also... if it's still the original engine... trying to push more HP will kill it.
With that said... if you have a fresh rebuild... you can squeak out another 15 hp (or so) without hurting the reliability too badly. But... that would be a pipe, head, and carb mods. That extra 15 hp will cost you about $1000... and it will net you about an extra 4~5 mph up top.