No need to replace every season. Portable, non-rechargeable extinguishers are to be replaced at a maximum of 12 years old. I just replaced the 2 Kidde 10-pounders on my bot. They were stamped 2007 on the bottom and still showed a full charge.
"Certification or licensing by the state or local jurisdiction as a fire extinguisher servicing agency should be required for annual maintenance of extinguishers with one exception. There are a number of approved small extinguishers that are not intended to be recharged (typically B-I rated extinguishers). The fire extinguisher label will clearly state that the extinguisher is non-refillable or non-rechargeable and is to be removed from service at a maximum interval of 12 years from the date of manufacture. Because these units cannot be recharged, the annual maintenance requirements do not require disassembly of the extinguisher. Due to the special responsibility of the owner or operator of a vessel for the safety of the crew and vessel, the USCG considers that the annual maintenance for non-
rechargeable extinguishers may be performed by those designated by the owner or operator. Thus, a qualified servicing agency is not required for maintenance of non-rechargeable extinguishers. However, a record of maintenance must be available for the use of the Coast Guard inspector."