U.S. Navy submarines are manned solely by volunteers from within the Navy. Because of the stressful environment aboard submarines, personnel are accepted only after rigorous testing and observation, as a consequence submariners have significantly lower mental hospitalization rates than surface ship personnel. Furthermore, submariners receive submarine duty incentive pay (SUBPAY) in addition to sea pay.
Some 5,000 officers and 55,000 enlisted sailors make up the submarine force. In addition to submarines, they are assigned to submarine tenders, submarine rescue ships, deep-diving submersibles, floating dry docks, shore support facilities, submarine staffs, and senior command staffs.
Until 2014, submariners worked an 18-hour day, as opposed to a standard 24-hour schedule. Sailors spent 6 hours on watch, 6 hours maintenance and training and 6 hours off (3 watches of 6 hours.) In 2014, the Navy began transitioning the fleet to a 24-hour schedule.
The submarine force has always been a small fraction of the active Navy. During World War II all submariners (including the rear echelon) accounted for less than two percent of Navy personnel, but accounted for 55 percent of Japan’s merchant marine losses. In 1998 only about seven percent of the Navy’s people were submariners, though they operated one-third of the Navy’s warships.
Source – https://en.wikipedia.org/wiki/Submarines_in_the_United_States_Navy