Could not get on plane

[BuilderFL]

Took the boat out yesterday for a test drive by myself and was working fine.
1. One person on board (200lbs)
2. Idle was steady at 800-850 rpm.
3. Throttled up real good and got her up to 39 mph at 5200 rpm.
Believing everything was good to go, I called the wife and kids for an inter-coastal night ride.
1. Six persons on board (965lbs)
2. Idle was steady at 800-850 rpm.
3. Could not get on plane, would not go beyond 7.6 mph and would not throttle more than 2200 rpm.
All other things equal.
I did notice that on neutral, the throttle would rev up to 5500 rpm but in gear did not have the power to move the boat.
Any ideas as to what’s going on?
Merc 200hp