before doing anything, what drive system do you have and how much power can it reliably handle is a very IMPORTANT question you should be asking yourself.
and with a boat, changing the prop is sometimes the best way to change the performance.
but if you do add HP, you MUST change the prop too. otherwise you risk overreving and damaging the motor.
the bigger a prop is (diameter and/or pitch) the more power it takes to spin it at high speeds. your stock motor can hit a certain max rpms with the current prop and no more, add power and you can start turning that prop a bit faster and overrev the motor.
if your current prop isnt the right one, it can be costing you fuel. too little pitch and your running high rpms but barely moving.
too much pitch and you cant get into the peak powerband where the motor runs at its most effecient, you might still have the speed, or you might not. a smaller prop would reach higher rpms and still push you forward just as fast with those higher rpms (imagine a fish tank filled with jello with a prop in it. that prop has a pitch of 15", one full revolution moves it forward 15". a 17" pitch prop goes 17". but spin that 15" just a few rpm faster then the 17" and both move the same distance in the same time).
1st, figure out what kind of power your drive can handle.
then, consider if a prop will get you enough change to make you happy.
if you dont think it will, a small increase in power plus a small increase in prop will definelty be noticeable.
i'd suggest the small and simple things. a set of roller rocker arms in your valve train with a slight increase in valve lift ratio. you'll reduce the friction in your valve train, power robbing friction, and reccover a little lost hp there, and the increased opening of your valves will help let more air in and out of the cylinder for another little increase.
you really dont want much more power. its not gonna help your fuel economy and its gonna increase the likelyhood of breakdowns.
BTW, if you really want to increase your fuel economy, get a boat that doesnt have a 454 big block motor in it.
with the added weight, most of the big block boats are NOT any faster then the small block boats.
same is true of the small block v8's compared the 4.3 v6. get fuel injection on that v6 and it gives you all the poower you'll need to move the boats that motor comes in while burning much less gas.
and if you really want a big increase in fuel effeciency without sacraficing much speed, go with an alum hull. alum is much lighter then fiberglass, an alum hull can hit the same speeds with much less power while consuming much less gas.
they do make some larger hulled alum boats with those same big motors. and they've got middle sized boats too, not impossible to have a big block in a boat that is barely over 20' long.
my boat is a 16 alum hull. 60 hp gets my going 40 mph, could go faster if i changed the prop and ditched the passengers and their fishing gear.
and a 6 gallon tank will last me longer then your 30 or so gallon tank lasts you. i go out 2 or 3 times for every time i get gas. i burn more gas in the truck getting to the lake, and thats just a 15 mile drive......