https://www.boatus.org/fire-extinguishers/#:~:text=NEW%20REGULATION
In particular for owners of older Boston Whaler boats, the following summary is applicable:
BostUS wrote:If you own a boat that is 2017 model year or older (between 1953-2017):
You may keep your extinguishers labeled B-I and B-II as long as they are still serviceable, but
If there is a date stamped on the bottle, extinguishers must not be more than 12 years old according to that date.
For my 1990 Boston Whaler REVENGE 22 boat, I discovered that I was no longer in compliance because the date stamped on the bottle was more than 12 years old.
The BoatUS organization also has the very good FAQ sheet:
https://www.boatus.org/fire-extinguishers/fire-faq.pdf
I am now in the market for a fire extinguisher rated 5B (or 5BC) that contains a statement of USCG Type Approval. Because the boat is less than 26-feet, only one extinguisher is needed. So far, the closest retail source looks like WALMART.
Cf: https://www.walmart.com/ip/Kidde-Auto-Marine-UL-Listed-Fire-Extinguisher-10-B-C-Rated/936046033
The link above is for a 10-BC rated, which has more capacity than the minimum required by the regulations and retails at $22.47. There is also a 5-BC model for $13.84 which should meet the regulations for boats less than 26-feet:
https://www.walmart.com/ip/Kidde-5BC-Fire-Extinguisher-Model-KD61W-5BC-KD61W-5BC/631439982
If preparing for boating next season, be sure to check the fire extinguishers on board to see if they comply with the new regulations that require them to be not older than 12-years-old.