What engine do you have?? My 19 footer has a 26 gallon tank. Check wiring, ground, and the two connections that go from the gauge to the sender on top of the tank. Mine was unhooked. Took about 5 seconds to fix.
If you have a square or rectangular tank, measure the dimensions--length, width, height. Come up with cubic inches.
Then, go on ******** or other places that sell square/rectangular fuel tanks, and they'll give you the dimensions and gallons. Compare cubic inches of the tanks and you should be able to compute the gallons.
If you have a tank under the hull, or a v shaped custom tank for your boat--your SOL. New tank floats and gauges are easily purchased--if you cannot trouble shoot your present gauge problem.