Art-- I woudl strongly recommend a visit to a reputable and experienced fire extinguisher company, particulary one with marine experience if possble.
We did this a number of years ago when we needed to have some of our extinguishers checked and recharged, and it proved to be very valuable. We learned a lot about extinguishers, what makes some brands good for boats and others not so much. And the company suggested extinguishers for the different compartments in our boat, for example using foam in the main cabin where all the electronics are as opposed to dry chemical.
We continue to use this company for our periodic extinguisher needs.