What does USS stand for (specifically the 2nd "s")?
United States Ship
http://www.history.navy.mil/faqs/faq63-1.htm
The prefix "USS," meaning "United States Ship," is used in official documents to identify a commissioned ship of the Navy. It applies to a ship while she is in commission. Before commissioning, or after decommissioning, she is referred to by name, with no prefix. Civilian-manned ships of the Military Sealift Command (MSC) are not commissioned ships; their status is "in service," rather than "in commission." They are, nonetheless, Navy ships in active national service, and the prefix "USNS" (United States Naval Ship) was adopted to identify them. Other Navy vessels classified as "in service" are simply identified by their name (if any) and hull number, with no prefix.
Into the early years of the 20th century there was no fixed form for Navy ship prefixes. Ships were rather haphazardly identified, in correspondence or documents, by their naval type (U.S. Frigate ____), their rig (United States Barque ____), or their function (United States Flag-Ship ______). They might also identify themselves as "the Frigate _____," or, simply, "Ship ______." The term "United States Ship," abbreviated "USS," is seen as early as the late 1790s; it was in frequent, but far from exclusive, use by the last half of the 19th century.
In 1907 President Theodore Roosevelt issued an Executive order that established the present usage:
In order that there shall be uniformity in the matter of designating naval vessels, it is hereby directed that the official designation of vessels of war, and other vessels of the Navy of the United States, shall be the name of such vessel, preceded by the words, United States Ship, or the letters U.S.S., and by no other words or letters.
--Executive Order 549, 8 January 1907.
Today's Navy Regulations define the classification and status of naval ships and craft:
1. The Chief of Naval Operations shall be responsible for ... the assignment of classification for administrative pur- poses to water-borne craft and the designation of status for each ship and service craft. ....
2. Commissioned vessels and craft shall be called "United States Ship" or "U.S.S."
3. Civilian manned ships, of the Military Sealift Command or other commands, designated "active status, in service" shall be called "United States Naval Ship" or "U.S.N.S."
4. Ships and service craft designated "active status, in service," except those described by paragraph 3 of this article, shall be referred to by name, when assigned, classification, and hull number (e.g., "HIGH POINT PCH-1" or "YOGN-8").
-- United States Navy Regulations, 1990, Article 0406.
Some, but apparently not all, other navies also use prefixes with their ships' names. Perhaps the best known of these is "HMS" (His or Her Majesty's Ship), long used by the Royal Navy. In earlier times this was also seen as "HBMS," for "His Britannic Majesty's Ship." British Empire/Commonwealth navies used their own versions of this, inserting their own nationalities, such as HMCS for Canada, HMNZS for New Zealand, or HMAS for Australia. The Royal Saudi Naval Forces also use "HMS." Argentina uses "ARA" (Armada de la Republic Argentina); the Philippine Navy identifies its ships as "BRP" (Barka ng Republika ng Pilipinas). The Imperial German Navy used "SMS" (Seine Majestäts Schiff); the World War II Kriegsmarine does not appear to have used a prefix, but the modern Bundesmarine uses "FGS" (Federal German Ship). India and Israel both use "INS" to mean Indian Naval Ship or Israeli Navy Ship. Lebanon and Tunisia, on the other hand, do not use any nationality prefix.