A common problem
In this case the gauge is a voltmeter and the float mechanism in the tank is essentially a variable resistor. Voltage goes to the tank sender, if the tank is full the circuit is almost open and the gauge reads full. As the float drops, resistance increases, and the needle drops correspondingly.If the problem is with the sender, it can be tested by removing the shelf in the starboard aft locker and removing all wires. Tested with an ohm meter it should read between 60 and 120 ohms. Open or closed circuit readings indicate a failure of the unit.If that checks out and, as it sounds like from your description, the gauge reads 'empty' when not working, there may be a broken wire or a short in the circuit.If the sender is good and the wiring is good, that just leaves the gauge to be bad. If you open the engine pod to do all this testing, jump the power lead from the lights to the terminal for the tank lead (pink) and the gauge will read 'full' when you turn the key on. If the gauge isstill on 'empty' get a new gauge from Seaward Products.