Not that familiar with that boat, but there will be a hatch, typically on the deck, that allows you access to the sender unit on top of the fuel tank. Most have two wires (ground and send), some only one (send - the tank is grounded independently.) Check the wiring there. If it seems good, remove the six screws holding the sending unit into the tank, pull the sending unit, clean it carefully, (it will probably be fairly crudy given its age) then attach an ohm meter between the sending wire and ground wire (or ground on the plate.) Work the float up and down over the range of the fuel. You should get a nice even variance in resistence. Normally, the higher the float, the less the resistence. If I recall way back in my brain cells, the resistence should vary between about 30 to 130 ohms - but best to look it up for your particular type of unit and gauge.
If all that is working properly, check your gauge and clean any corrosion off the terminals. When all is working well, you should be able to move the float up and down and see a corresponding change in your gauge. So, if you get a good range from your ohm meter, but not a good range on your gauge. You have a faulty gauge or faulty wiring between the gauge and the sending unit. If you have the type of sender that has a float on the end of a long arm, then you may be able to adjust the arm range to better match your actual fuel level in the tank.
Having said all of this, I have never seen an in-tank fuel gauge that is very accurate. I have always installed fuel flow meters on my boats so I can get a very accurate reading of total fuel usage from each fillup. Fortunately, my Honda BF225 has fuel flow and usage built in, and that has always read within +/- 2.0 gallons of actual usage when I fill up at 80 to 100 gallons. That's darn good.
