1) Sending unit is stuck or broken
2) Wire between the tank and guage is broken or shorted
3) gauge is broken.
Dig down to where you can get to the top of the gas tank. You should be able to find the sending unit for the gauge, it will most likely have one wire coming off it. Disconnect the wire from the sending unit. Holding the wire so it doesn't touch anything, turn the ignition on and observe the gauge. It should read either full or empty at this point (I am not sure which, but I believe it would read VERY empty). Now touch the wire to a ground (rig a wire to the engine block or the negative (-) terminal of the battery.) The gauge should go to the opposite extreme (VERY FULL).
If that works as expected then the sending unit is bad OR the ground wire to the tank has been lost. If it is a metal tank then you should be able to make the gauge swing by touching the wire to the tank. If it doesn't then you have a bad ground wire. If it does, then pull the sending unit out (5 or 6 small screws in a circle). You will probably find that the mechanical system has broken.
Sending units are not very expensive and available at many boat shops. Measure the height of the tank so that you know what size to buy. The new sending unit will need to be adjusted the height, instructions should be included.
If touching the wire doesn't do anything then crawl under the dash and find the sending unit wire on the guage. Disconnect the wire, then attach a test wire. Watch the gauge as you touch the free end of the test wire to ground, the needle should go between very empty to very full (probably violently so don't over test!).
If there is still no change in the guage then repace it. If it is a standard 2" round gauge then replacements are easy to come by. If it is some fancy style that matches the boats dash scheme then you will probably need to go to the dealer.
If the gauge operates properly with the test wire then it is likely that the wire from the sending unit to the gauge is bad. You can try to follow it and find where it has been broken, pinched or otherwise damaged. Or you can just run a new wire.