Why is your 2006 Yamaha fx ho waverunner stuck in low rpm mode apparently because it wont run above 6500 rpm despite me taking in out of low rpm mode with the remote control?

The engine may only be able to go to 6500rpm due to how loose the throttle cable might be. Most watercraft will gain 200rpm to 2000rpm just by properly adjusting the throttle cable at the carburetor.

There are other factors that can limit speed like: Engine wear - Filters (dirty/clogged) - Fuel (wrong octane, dirty) - Spark plug (wrong gap, wrong type, worn out) - Damaged propulsion (propeller, propeller housing, intake, outlet), - Carburetor settings (jets, mixture) - Coil (worn out, corroded terminals)