Assuming your tacho is fairly standard it will have 3 wires going into the back to make it work (Ground (usually black), a +12V feed which will be live when the engine is "running", and a "signal" wire). You may also have a +12V feed which goes to the "backlight" which may or may not be connected and may or may not be wired as always on, or e.g. on when the nav lights are on.
Assuming you have those wires (iboats.com or your owners manual will be able to help with colours), and they are in good condition then I would put a multimeter between the signal wire and the ground - with the multimeter set to AC (a 20V a.c. range should be fine). With the engine running this should show a voltage (and zero with it off). The signal will increase with the revs. If it does this and all the wires and connected then it is probably your gauge that is faulty - or set completely wrongly, as T.D. suggests there is usually a "dial" or setting of some sort on the back to adjust how many poles etc it should use in its RPM calculation.