Powder released inside a small room will rapidly give zero visibility.
Powder will also give a lot of cleaning up afterwards.
So if you can put out the fire by other means - do it.
Fire blanket and/or fire spray can be useful to put out a small fire - less cleaning up.
I have used fire extinguisher once to put out a fire inside a boat..
An oil burning oven had a fault in the burner and started a fire.
I used the first bursts of powder to put out the fire.
In the reduced visibility i spotted a secondary fire above the oven and used the rest of the powder to fight this fire - but it would not die..
Then I realized - I had been trying to put out an electric light bulb...
How was the cleanup afterwords? Was the powder all throughout the boat?
This survey finding has caused me to do a lot of research on fire extinguishers and I must say I'm learning a lot. I also suddenly feel remiss in my previous plan of just having one more than the required number of extinguishers because they are all of the powder type. Sure, it may put out the fire, but the video that Minnewaska linked to really opened my eyes to not just the major mess issue, but the zero visibility issue which I think is a huge problem.
If you haven't watched the video yet, drop what you're doing and watch it NOW!
As Knuterikt experienced, you will loose your situational awareness nearly instantly, and if the fire flares up, is in more than one location or is complex somehow you may not have the visibility to find it. This is not good! Imagine an electrical fire where you put out the flames, and now don't have the visibility to turn off the electrics, or find the issue and isolate it.
So, I've been reading about all the different options for extinguishers and I think that I'll probably end up spending the extra money for genuine Halon. Halotron seems like a decent alternative but Halon is apparently twice as effective and not twice as expensive as Halotron. I like the idea of unmatched effectiveness, ease of cleanup (so no hesitation to use it) and safety in confined spaces.
C02 is clean, and has the advantage of being able to safely shut down a runaway diesel engine, but when concentrated Co2 gets toxic. I'm not sure if a bottle of CO2 discharged in a boat cabin would reach toxic levels. With Halon and Halotron you don't have to worry about it. Halon is what's recommended for aircraft, and I hear aircraft fires are a no-no as well.
I wonder if Halon would safely shut down a runaway diesel engine if sprayed into the air intake?