Now all these years on, I'm afraid to say really that there is no real solution. You can search for your own one if you like. It was a bit of a joke. I honestly didn't really have a specific idea in mind. The use of the word conjures up more 2000 Leagues Under The Sea images than those of a nuclear powered submarine, but nevertheless the association of brass with the sea is inescapable.
Water means a hull breach, and pressure loss, and everyone dies.
Fire means all the oxygen aboard burns and everyone dies. The final proof that the answer to the riddle is, truly, a submarine lies in the music. As in German U-boats, because the music itself doesn't sound Germanic. If you listen to Dodo, Lurker, and Submarine back to back, you will find that Lurker has some of the exact same drum lines, and Lurker ends on the same music that Submarine begins. Like the suites of thematically related music in A Trick Of The Tail and Wind and Wuthering, this is yet another case of Genesis splicing and dicing a long track of related music into separate parts scattered all over the place.