Has it ever run faster?
If so, then check if the bottom is dirty, if the prop is damaged, or if you have prop slip from a spun hub. To check the last item, while at rest shift into gear and then gun the motor. If the revs increase faster than the speed does, you may have a spun hub.
There are other issues that could account for your lack of top end speed. There could be a fuel delivery problem, a spark issue or other engine ailments.
Now if the boat always had this issue, you need to provide more data to this forum. We need to know engine model, prop diameter, prop pitch, whether there is any sort of an engine bracket (probably not).
Also tell us how you are measuring speed, GPS speed or (ugh) the speedo gauge.
