They needed a cigarette lighter so they could smoke their cigarettes. So they took one of the 4 cigarettes and threw it overboard making the entire boat a cigarette lighter. It's a play on words. Lighter, as in a fire starter and lighter, as in weighing less.
In sea terms, when something goes overboard, the boat becomes lighter (weight). So for example if an 1 apple goes overboard, then the boat becomes 1 apple lighter.
Hope that helps!
Hope that helps!