There is a lot of confusion surrounding the use, testing and inspection of fire fighting equipment on boats. The rules for commercial passenger carrying boats are different to those used in the recreational market and in summary these are that non-rechargeable extinguishers have a life expectancy of 12 years after which time they are replaced. There is normally a two number date stamped onto the bottom of the cylinder 16 for 2016, 98 for 1998 and so forth. After 12 years they need to be replaced. However they need not be inspected by anyone other than the boat owner to ensure that they are in correct and proper order. The full transcript is attached below in the PDF pulled from the USCG website. This standard and most of the others relating to fire are jointly drafted by the USCG and National Fire Protection Agency, often just called the NFPA.
However if your boat has an installed fixed system in the engine room then this can only be inspected by a certified person and must bear a tag showing the date, the inspectors initials and the certified company that he works for (these resemble old fashioned luggage tags).
Link to inspection requirements
As I mentioned the rules for commercial boats are more strict, and this includes skippers with a 6-pack license who occasionally charter their boats, these must be inspected by a certified person each year.