Few boats have an accurate dip stick for their fuel (or water) tanks. These are easy to make if you have a bit of time. Start out with a half inch dameter wooden dowel, which will fit into an access hatch or fill for the tank in question (we typically have a stand pipe welded to our tank tops, which is headed with a ball valve and a threaded cap).
Start with an empty tank, and then pause every 20 or 25 gallons to allow the fuel to level out and any foam to subside (you may have to wait 15 to 20 minutes). Drop the stick into the tank, note the liquid level, and mark it with a pen.
When you have finished the process, give the dip stick a clean, and seal with epoxy or varnish.