How many fire extinguishers are requires for an 18 foot powerboat with installed fuel tanks?

At least one USCG-approved Class B-1 fire extinguisher is required on any powerboat, unless it is under the following exception:

• "motorboats less than 26 feet in length,
• propelled by outboard motors and
• not carrying passengers for hire, need not carry such portable fire extinguishers if
• the construction of such motorboats will not permit the entrapment of explosive or flammable gases or vapors" [bullets added]

A UL-listed 5-B:C extinguisher, with 2 pounds of dry chemical, may provide the basis for the extinguisher, but it must ALSO conform to USCG-approvals (proper corrosion protection, labeling, mounting bracket, etc).