First before answering the posters questions there are some relevant questions:
What type of
boat is it?
How big (length) is it.
Is it gas or
diesel?
Where is the
boat located? (the country, you don't need to be any more specific than that)
Is it a recreational boat, or does it carry passengers for hire (this includes 6 or less, commonly called 6 pack, which are usually recreational boats)
All of these determine what rules apply. A recreational boat has vastly different requirements than a passenger vessel. A small recreational boat 16 to 26 ft has different requirements than a 45 or 50 foot boat.
Plus that, as someone said
insurance companies can require what ever they want. It doesn't make it a reasonable request. Try to get some clarification from the
insurance company or agent.
Now for you remark about
buying from West
Marine or other
Marine supply, yes that is what most people do. They buy the extinguishers they need and install them
DIY.