http://en.wikipedia.org/wiki/Captain_(United_States)#U.S._Navy.2C_U.S._Coast_Guard.2C_U.S._Public_Health_Service.2C_and_National_Oceanic_and_Atmospheric_Administration

Navy captains with sea commands in the Surface Warfare community generally command ships of cruiser size or larger. The more senior the officer, the larger the ship. In the submarine community, a captain will typically command a ballistic-missile submarine (SSBN), or a squadron of attack submarines (SSN).