She was commissioned into the Royal Navy in February 1943 as HMS J-826 and assigned to active service in the Mediterranean Sea, based in Malta, and was reclassified as BYMS-2026 in 1944.

On 1 August 1947 she was formally handed back to the US Navy and then struck from the US Naval Register, remaining in lay-up.

[4] The British millionaire and former Member of Parliament (MP), Thomas Loel Guinness bought Calypso in July 1950.

Cousteau restructured and transformed the ship into an expedition vessel and support base for diving, filming and oceanographic research.

[7] Calypso carried advanced equipment, including one- and two-man mini submarines developed by Cousteau, diving saucers, and underwater scooters.

The ship was also fitted with a see-through "nose" and an observation chamber 3 metres (9.8 ft) below the waterline, and was modified to house scientific equipment and a helicopter pad.

[citation needed] In 2006 Loel Guinness transferred ownership of Calypso to the Cousteau Society for the sum of one euro.