Fishermanbb said:
Could be one of several issues but it is most likely the sending unit. Also could be a stuck float. Both are very easy to check via the access plate in the deck.
If you tank is full and you unscrew the sender flange, gas will pour into the bilge!
I assume your float is stuck also. Sometimes just running the boat and bouncing around will un stick it but that doesn't work all that often.
To check the guage, open the access plate at the rear of the deck. You will see the sender plate and wires. Undo the pink or purple wire that goes to the sender (not the ground tab) and using a piece of wire to a good ground, ground the wire. Turn on the ignition and look at the gauge. It should read full. Now disconnect the ground leaving the wire just floating
and the gauge should read empty. If that works the gauge and wiring are good and the sender is probably bad. On rare occasions, the tank ground is bad but that will usually result in a false empty reading, not one bar.