Motorboat fuel tanks have a reserve as well. It's just a raised area that splits the bottom of the tank into 2 separate concave areas. One of the concave areas contains the end of the fuel line, and the other doesn't. When you run out of gas, you tip the tank up to dump the remaining gas from the other basin into the main one, and then you restart the engine (or keep it from stopping at all if you're quick enough on the draw) and head for the docks.