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 is a very soft, dark gray mineral with a semi-metallic luster. It has a greasy feel and you can write on a piece of paper with it. It is this last physical property that is responsible for this mineral being used in pencils.