The Discworld equivalent of our world's submarine, invented by Leonard da Quirm. At the time of use during the Ankh-Morpork/Klatch conflict (see Jingo), the Device was manned by Leonard, Lord Vetinari, Sergeant Colon, and Corporal Nobbs. After considering paddles, oars and even some kind of screw, da Quirm solved the problem of propulsion by imitating nature and using a moving fishtail, like a dolphins'. The device is usually powered by pedaling, but also has a large, screw-threaded auger for boring into a ship's hull just very slightly for attachment, so that the Device gets towed as the ship sails. Leonard was very shocked when Corporal Nobbs suggested that this auger could be used to bore clean through the hull and sink a ship. The Going-Under-The-Water-Safely Device is another fine example for showing that Leonard's genius does not cover the field of creative naming.
In a different continuinuinuum of a different world, in Shea and Wilson's Illuminatus! trilogy, the crew of the free submarine the Leif Eriksson are faced with averting superpower war, over the hitherto unnoticed and disregarded African island of Fernando Poo.
This is where the similarity ends, as Leonard's simple, functional, craft cannot be compared to what amounts to a nuclear-powered luxury liner, which according to one theory, was originally stolen from the U.S. Navy by arch-manipulator Hagbard Celine and rebuilt to his standards of luxury and comfort, whilst still retaining a full weapons load. And Leonard's boat, like Hagbard's, should really be painted bright yellow... I don't believe we're told this detail in Jingo.