Pontoon motor runs and goes in forward and reverse but will not go faster when engine revs up. Why???

by Mike
(Southwest Michigan)

We have a 24' 1987 Funtoon Pontoon. It has a 25 horse Mariner motor. The boat refuses to speed up in forward or reverse even though the motor revs up like normal. Very confused with the problem. Revs up, goes in gear, but with no speed increase. Any diagnoses would be much appreciated. Thanks