I don't recall ever hearing a news report about a ship sinking off our coast due to fire. The most recent incident of a fire at sea, that I'm able to recall, was last year when Cape Town's Fire and Rescue services boarded a cargo ship near Britannia Bay (map) to help extinguish a fire that had erupted in the ship's engine room. It took many hours of work under difficult conditions, but fortunately the ship and all it's crew were saved and towed back to Table Bay Harbour.
Imagine how scary it must be to have your ship catch alight while at sea. One would think that with all the water around it would be easy to extinguish a fire - but with the right flammable materials and without the necessary equipment - it's sometimes impossible to stop a fire before it causes the ship to sink.
I imagine that this door would lead to the engine room, and that opening it during a fire would supply oxygen to the flames, causing them to burn more furiously.