Try the boat with out the hatch so fresh air come in the carb.If this solve the problem you have to do some holes in the hatch or boat deck.
I have two of these boats,one with 23cc and Octura X467 it runs 66mph.
The other is a 43"Stinger is the same boat with different hatch,with 26cc and Octura X470 it runs 73mph.In the begining of these project I have the same problem.I solve the problem with a Perko clam shell ventilator mounted in the deck 5 to 6 inches front away in the carb side of the boat.You can buy these in any marine store.