What is requirement of fire extinguisher on a boat?

Powerboat, unless it is a small, open, outboard (Class A or Class 1) that otherwise qualifies for exemption.

At least one USCG-approved Class B-1 fire extinguisher is required on any powerboat with a permanently installed fuel tank or carrying passengers for hire.

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).

A 4-pound CO2 extinguisher or 6 quarts foam extinguisher may also be rated for USCG B-1 service.

Powerboats over 26 feet may require additional extinguishers, as detailed in the related questions.