That's really the only way to find out how much gas it holds. Even then, what does knowing it's capacity do to help reduce the concern of running out of gas? Unless you know your rate of consumption, distance travelled etc, you're not going to have much idea when you need to refuel. Unless the wiring on your boat is a butchered mess, odds are, your problem is the sender. Fuel senders are widely available and they tend to be standardized in both fit and function. You usually just have to remove the old sender, measure the tank depth, and cut the new sender rod to the length shown on the back of the package.
