Ankh-Morpork and its neighbours all have at least a residual standing army. Ankh-Morpork even boasts a Navy. At least, in theory, as in Jingo we see that its only extant battleship the Mary-Jane, needs a little bit of a refit before she can go to sea again. To be scrupulously honest, she needs refloating from her current resting place on the bed of the harbour. By default the Mary-Jane is flagship of the Fleet, which might in its day (presumed that of one of the last Kings) have been prefixed H.A.M.M.S.
To borrow an acronym from Roundworld naval terminology, in fact, the whole of the Ankhian Navy is STUFT. Every ship in the fleet that set out to Klatch, when serious football broke out over the elusive brigadoon island of Leshp. had to be borrowed, impressed or hired. On the day, the flagship of the Fleet was the unfortunately named Prid of Ankh-Morpork. Other ships referenced are the Indestructible and the Indolence.
As Samuel Vimes noted, the monarchy is long dead, but lies unquiet in its grave, and there are ghosts of its presence everywhere. Who knows? The H.A.M.M.S. prefix may still linger.... (His (or indeed, Her) Ankh-Morporkian Majesty's Ship, should there be wondering.)
One possibly Naval officer who appears in Night Watch was the hapless Sub-Lieutenant Harrap, who is tasked with discharging an unpleasant duty.
In an earlier age, the war navies of Ephebe, Djelibeybi and Tsort all ganged up on Omnia. The man in charge of the Djelibeybian fleet at this time was Admiral Rham-ap-Efan.
STUFT. During the Falklands War of 1982, the Royal Navy set to sea with an interestingly ramshackle fleet. Successive defence cuts had denuded the Navy of many of its specialist auxiliary and support vessels: modern defence thinking held that suitable ships might be impressed or borrowed or chartered from the merchant marine, in time of emergency, to make up the shortfall.
Therefore, the luxury liner QE2 rubbed gunwales with things like cross-Channel ferries of the "roll-on, roll-off type. These latter were so appallingly badly designed that a year or two later, one such, the Herald of Free Enterprise, sank in the Channel with considerable loss of life. The acronym STUFT - Ships Taken Up From Trade - wholly applied here.
The Mary-Jane: refers to Henry VIII's flagship, the Mary-Rose, that famously sank in home waters during a skirmish with the French, due to being top-heavy, dangerously overloaded, and with lower gunports opening a mere foot or so above the waterline.
Mary-Jane is also a universal euphemism for cannabis. A consolation to the crew, a reference to the hemp rope that rigged her (which sailors the world over dried, grated and smoked). Or the reason for her sinking?