All of this advice so far depends on a couple of things, 1.) Tach is accurate and 2.) Speed is from GPS or other accurate gauge. With the numbers you gave, you are at about 11% slip I think. The 21p you are running now may not be behaving like a 21, with that much slip, and allows the engine to rev higher but with loss of thrust up top.
For a no cost temporary-to-permanent solution, maybe just ease back on the throttle, since you are that close to optimum. Only saying that because you have only mentioned RPM's as the issue you want to address. You also have not specified what brand or material the current prop is. Or the condition.
How is the holeshot with this one? Do you tow tubes or skiers/wakeboarders? Is highest top speed attainable one of your goals?
So there is room for improvement, to get the slippage down, which will increase top speed some, and take some RPM's off the top. A 4blade with same to same brand and material in the same pitch would probably get you there. If you tow skiers then it is probably the next thing to try out.
If top speed is a goal then going with a more aggressive 3blade prop in same pitch is another approach. If the current prop is aluminum go to the same pitch in a stainless. If it is stainless, you may need to try a more aggressive prop like an Enertia or similar at that same pitch.
Good luck!
