Where do you store fire extinguishers on a boat?

in a readily accessible location

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EXACTLY, if you see a fire, you need to be able to get at the extinguisher easily so it must be in an open area and not covered up or in a cupboard or storage unit. You could also consider where fires are likely to happen - near a cooker, by the engines etc. It is also a good idea to have one near a door so you can pick one up as you enter or leave a cabin.