As I said, magnesium will decompose water. Unless you're trying to burn it underwater, though, that's different from "producing its own oxygen". BTW, does jettisoning a burning aircraft pose any risk of a hydrogen explosion above the surface, or is the hydrogen too diffuse at that point to pose a problem?
Not sure we never had a delta fire and I wasn't a flight deck crewman. I was a fire fighter while the ship was in port or in the yards not during flight opps. Flightdeck had a specially trained fire fighting crew of its own at sea. I've was told if you tossed a helo it would keep on burning even on the sea floor. Better I guess to have a surface blast risk than a hole burned down to the 6th deck. We did have cranes for that purpose and the guys wore asbostos suits. A demenstration grease vat fire and a cup of water poured on it is something you don't forget though :>}