Here is a statistic you should know. Those 10-BC dry chemical fire extinguishers we all carry (designated B-I by the Coast Guard) will discharge for maybe 10 seconds. For an onboard fire with a head start, that won’t be long enough to put the fire out. Then what? If your boat is larger than 26 feet, Coast Guard regulations require a second B-I extinguisher aboard, that or a larger B-II. A second 10-BC gives you another 9-second shot-if the fire hasn’t put this second unit out of reach. A B-II extinguisher is better, carrying at least a 60-BC rating, which doesn’t mean the extinguisher will discharge for six times as long, but does mean it has six times the extinguishing capacity. But if your single B-II is on the other side of the flames you won’t be able to get to it, so think carefully about where you’ll mount them and consider having two available.
Meeting Coast Guard requirements equips your boat to extinguish only the smallest of onboard fires. For real fire protection, take aboard B-II extinguishers, or at least the larger B-I, rated 40-BC. All but the smallest watercraft should have at least two extinguishers aboard, mounted in opposite ends of the boat.