Final answer:
The US Coast Guard mandates a Coast Guard-approved B-1 type fire extinguisher for Personal Watercrafts, which is suitable for flammable liquids such as gasoline.
Step-by-step explanation:
The type of fire extinguisher that the US Coast Guard requires to be on board a Personal Watercraft (PWC) is a Coast Guard-approved B-1 type. These extinguishers are designed to fight class B fires, which involve flammable liquids like gasoline, which is particularly pertinent to PWC and boating scenarios. PWC operators should ensure that they have a fire extinguisher on board that is in good working order, readily accessible, and within the valid period for its intended use in the event of an emergency on the water.