How's the exhaust too? Sometimes old transfer hoses to the out-drive can come apart internally and partially block flow... If they've been on there 10 years, I'd just do new hoses. They don't need to be Merc, they can usually be Gates Green Stripe in the appropriate size and with with new all SS clamps. I usually double clamp top and bottom...
OK, for speed or acceleration - the prop is the biggest thing assuming the motor is optimal as checks against all the above listed items... OH, and you might want to check the coolant op temp. A cold motor will not make the power either...
So as the boat comes up and the motor starts to spin it all, the prop edges get more and more important. As RPM increases, prop condition like rotational drag goes up with the square of blade speed. That's why guys chasing speed and fuel efficiency are freaks about prop condition.
I know it's a family boat for some summer fun and all. But it's like tires on a performance car. It will not perform w/o the best prop for the load. The best prop will usually be a three blade calc'd and checked with the family on board and full fuel, cooler with some beer, etc. It should load a well tuned motor to say 100 RPM from max efficiency point. I think yours is like 4,700 or so. So all family loaded on plane with no headwind, maybe 4,600 or there abouts.
Once it's just you or you and wife or daughter, it will pick up that last 100 RPM and be a bit faster.
The most efficient props are the razor sharp SS props with thin blades and a speed cup. That cupped edge does nothing down low, but when you are on the last bit of motor and out on the lite side of loaded, it can be worth 2~3 mph.
So do you have a Tach? To really figure this out, you need a well tuned motor and you need to see the max RPM under load...
Take a small file (half round & flat) and dress the edges of the existing prop to get rid of bumps and burr's. If it has a real bend in the edge, Hammer it back with a backing block (piece of flat Oak with rag to hep soften the shock, and then file flat.
Sand it down and paint it with some engine paint. Not for slippery, but so we can see what happens after the next run.
If there are any cracked blades - forget it. Buy another prop, or send it in to be welded and re-pitched... Then start the hunt for the optimum
you get it tuned right and propped right and it will drive well and be more fun