In World War I, British submariners referred to themselves as being in "the Trade". I recently read an explanation: Hidebound, spic and span surface navy officers thought their brethren in the submarine business sloppy and scruffily dressed. In fact, indistinguishable from plumber's apprentices in appearance. Hence "the trade".
I doubt if Max Horton, one of the earliest and most successful WW I submarine commanders, would have liked the comparison. Here he is in later life as Sir Max Horton, Commander-in-Chief Western Approaches, the man most responsible for Atlantic convoys and the eventual triumph over the U-boats in WW II.