No, it isn't a linkage problem. The linkage opens or closes a valve, it doesn't operate the clutch directly.
The first thing you should try is to change the transmission fluid. It is possible that the oil has just gotten so dirty/gunky that it is causing a lot of drag as the clutch plates rotate.
If that doesn't fix the problem then I am afraid it won't be an easy fix. The likely cause of the drag is that one or more of the clutch plates have warped and they are not totally releasing. The news gets worse: the warpage would have been caused by the clutch plates overheating, and they propably overheated because they were slipping, and they were probably slipping because they were not able to withstand the load.
If you were recently doing something unusual (like towing a large boat) that could have resulted in the transmission slipping, and you are not likely to do that thing again, then you might be okay except for the annoyance. Just be sure to kill the engine whenever there is anyone in the water next to the boat.
If you have been doing your usual riding then I would have doubts as to how long that transmission is going to last.