Do you have a tachometer on the boat?
If not we will have to assume that your current 21 pitch prop was "fitted" correctly for your rig.
If the motor is labouring trying to get going that is a fair indication that you have "too much" pitch unless the motor needs a good tune up.
The fin is there to help get the boat up on plane quicker - it places quite a bit of stress on the cavitation plate of the motor and does make it more difficult to steer in turns (which puts added stress on the steering system), but does have some limited usefulness if you travel long distances through canals or no wake zones by keeping the bow down enough that you have good visibility forward).
Contrary to the claims of the mfg's of "fins", they do not add any "noticable" improvement to top speed or fuel economy.
(should have added)
Off the top of my head, a ski style boat in the 17/18 foot range with a 90-115 horse motor would generally run a 17 pitch prop - maybe a 19 if the boat was aluminum and possibly down to a 15 if it a heavy fiberglass build - so unless you have a light 15 footer, 21 sounds quite "high"...
It sounds like a previous owner was trying to address a "percieved" sub-standard performance from the rig. Which means that you could be "over propped" as well.
There is a myth out there that "More Pitch = More Speed" - that is only true if you were running way too little pitch in the first place - otherwise, more pitch just causes the motor to bog and can do significant damage to the engine.
So before we can really advise on a prop, need a couple pieces of information -
does the boat have a tachometer? if so, what rpms do you get at full throttle?
(if not)
what is the max recommended horsepower for the boat? (look for the coast guard plate)
if you can't find that - what is the length of the boat? what model? (ie sunray 220 or whatever)
Without a tach, which is really the only way to properly determine "what prop" should go on "your" boat (identical boats with the same motor will run different props, there is no "one size fits all"), we will have to try and figure out what you should "maybe" be running in general...