It is often suggested that you have at least one more fire extinguisher than mandated for your type/size of vessel.
A throwable PFD is good idea for any vessel (except maybe kayaks). If anyone wants to go for a swim you can tie a dock line to it so the swimmer can rest and neither of them will drift too far away.
My personal rule on PFD's is:
When boating alone, everyone wears a PFD.
