The boat can only slow down if the gas in not well supplied to the engine. If there is a cutoff of gas to the engine, it might slow down and come back later after the gas is sufficiently supplied. I would recommend that you take the boast to a mechanic for a check up.
I agree it could be a fuel supply problem. That could lay in the fuel pump or the filters, but also in the injector pump or its control electronic. Work on the injector pump should rather be left to a qualified mechanic.