Sounds like you have a bad fuel sender. Most marine fuel senders read 30 ohms full and 240 ohms empty. If you have this type a simple test is to disconnect the wire that is mounted to the center stud of the sender and connect it to ground. Now turn the key on and if the gauge is good it should read full. Usually you can just remove one of the screws that hold the sender in the tank and attach the wire under the screw. Do not turn the key on and then go back and touch the wire to ground as you may cause an arc and if there are fumes present.... well you get the picture. The other way to test the sender is to measure between ground and the center stud of the sender when the tank is full and empty with a multimeter. If your readings are + or - 10 from the numbers above you can usually bet that the sender is ok.
Fuel senders and gauges in boats are usually pretty low quality. Normally a low bidder wins the contract type thing. You can usually get a high quality sender to replace your oem unit and be able to trust your gauge for years.
If you need more info, email me and I would be happy to help.