Gasoline fumes are heavier than air. Fuel and fumes on board need only a spark to explode or start a blaze. Fires need three things: heat (such as a match or spark from the ignition), fuel (gasoline, propane, etc.), and oxygen (air). Removing any of the aforementioned fire elements can extinguish the fire.
IF YOUR BOAT IS UNDERWAY AND A FIRE STARTS:
Stop the engine immediately - this should always be your FIRST action!
Position the boat - so the wind will blow the fire away from the boat.
Try to separate the fuel source from the fire.
Grab the extinguisher.
USING A FIRE EXTINGUISHER
Aim the extinguisher at the base of the fire.
Pull the pin and squeeze the two levers together.
Use a sweeping motion with the extinguisher while maintaining focus on the base of the fire.
Continue to extinguish until the fire is COMPLETELY out.