When in port, crew members wear regular Navy uniforms. At sea, members wear one-piece blue coveralls called "poopy suits." They are very comfortable to wear and reduces the number of clothes the sailor has to bring to sea. Submarine crews usually wear sneakers or other soft bottomed shoes when at sea, as sound quieting and stealth are always foremost in a submariner's mind.
October 31, 2013
From the Navy's FAQ about submarines: Question #26 — out of 61 — "What clothes do you wear?"