Your 14X36 "boat will plane with 4people on it" and a 20hp longtail?
That not too bad ... well it's miraculous !
Depending on your tach -=-- depends if it's direct driven or impulse, etc. They can be quirky if impulse (ie has a wire that wraps around the spark plug and has a digital readout). Some little impulse tachs can have a built in delayed response that can mess-you-up depending on how you read it. Some kinda have to be stared at a long while until they 'even out', or they will stutter 300-500 rpms.
Governors (and rev limiters) are a huge mystery that should be studied per the individual motor.
Never mess with "adjusting" a governor unless you know exactly how YOURS works.
They can adjust by as simple a means as a stop screw, OR switching holes for the springs on their "arms", BUT be sure to find out how yours exactly works FIRST.
They vary a lot from engine to engine , and often have a touchy super-sensitive relationship with the carb throttle.
All that is why they can be the first source of rpm problems.
"Can i Just get a large pitch prop and connct my throttle right to the carb since I dont have to worry abou over-revving?" .....
If your tail hits an object and hops out of the water how you gonna keep the revs down?
Those final rpms and gps speeds will always be limited by the fact that engine needs to keep from thrashing its' flywheel, or floating its valves, or breaking its connecting rods or.....