The pulse pickup on the back of an alternator is often marked with a "W".
If the belt is slipping the tacho reads too low.
As said in above post, use a handheld laser tachometer to measure actual RPM. If the actual RPM is lower than your engine panel indicates, than maybe the size of a pulley have been changed before you owned the boat
Some tachometers can be adjusted on the rear to allow for a change in pulley size.