At Havasu where the elevation is roughly 700ft, try the "factory" prop first. Note RPM's at wide open throttle with the boat trimmed up
on plane. The RPM's should fall within the recommended RPM range which should be listed on the plastic engine cover
or on the small electrical box on the engine. IIRC it should be between 4400 and 4800 RPM's but check the data on the engine.
Be aware that engines will lose some HP when the ambient air temps go up. An engine rated at say 260HP on a 70 degree day
at sea level will make less power when the air temp goes up like it does at Havasu. When it's 110 degrees the engine will
lose HP sometimes to the tune of 30 or 40HP.
RPM's (detonation), coolant temp and oil pressure are really all that matter in regards to boat engines. Pay no attention to the speedometer.