During 1940, "Beaver" helped establish a submarine base at Gregerie Channel in St. Thomas, Virgin Islands, and operated at the seaplane base near Hamilton, Bermuda. She also served as temporary flagship for Atlantic Fleet submarines before becoming the tender for SubRon 7 based at New London in April. Following American entry into World War II on 7 December 1941, "Beaver" led 11 R-boats to the seaplane base in Bermuda, arriving there on the 10th. For the next nine months, the tender alternated between Bermuda and New London, supporting submarine patrols along the Caribbean-Bermuda-New England shipping lanes and assisting antisubmarine training for American destroyers.