Depending on the size of your boat you may need more than one. Boats less than 26' have to have at least one B-1 fire extinguisher on board. Boats 26'-40' need to have at least two B-1 fire extinguishers on board.
Graphite is an electric conductor, consequently, useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers (a phenomenon called aromaticity).
Graphite, while also composed of carbon, has an entirely different crystal structure, and different uses. Graphite is used as an electrical contactor (a brush) in electrical motors. It makes a great dry lubricant for things like locks, and when mixed with clay, forms the lead for ordinary pencils.