What i'm thinking is either the engine is starving for fuel, or there is something going on inside of the carbs, and that can include being dirty, or having a sticking float.
What I would do is run the boat again, and have a helper drive while you pump the primer bulb over and over as fast as you can.
1. If the primer bulb sucks flat, or will not easily reinflate. The fuel tank is either building up a vacuum, or there is a blockage of some sort in the tank, fuel line, or the bulb itself is bad.
2. If pumping the bulb allows the engine to run better, the fuel pump is weak and not keeping up.
3. If no difference, the problem is going to be in the carbs, and they would need to come off and be rebuilt.
Does that all make sense? If yes, did you have any further questions on the issue?