Unimak (AVP-31) was laid down on 15 February 1942 at Harbor Island, near Seattle, Wash., by the Associated Shipbuilders, Inc., launched on 27 May 1942; sponsored by Mrs. H. B. Berry, the wife of Capt. H. B. Berry, the personnel officer of the 13th Naval District, and commissioned on 31 December 1943, Comdr. Hilfort C. Owen in command.
Following shakedown and fitting-out into late January 1944, the small seaplane tender departed San Diego, Calif., on 20 March, bound for the Canal Zone. Arriving at Balboa eight days later, the seaplane tender operated on the Pacific coast of Central America into April, providing logistics support to advanced seaplane bases at Santa Elena Bay, Ecuador, and at Aeolian Bay, Battra Island, in the Galapagos group. She soon shifted to Coco Solo on the Caribbean side of the Canal and transported men and materiel to Barranquillas Colombia, arriving there on 25 April.
After escorting SS Genevieve Lykes back to Coco Solo on 23 and 24 June, Unimak conducted routine exercises with patrol planes into July. On 4 July, she received reports that a tanker near her position had been torpedoed and headed for the damaged ship. When she arrived on the scene late that day, she found the tanker still underway, making for the Panama coast. She immediately commenced screening the disabled ship and, aided by an escort of Army and Navy planes, shepherded the tanker safely to Colon late on the following afternoon.
Soon thereafter, Unimak shaped her course towards the last reported position of Navy blimp K-58. At 1532 on 9 July the seaplane tender sighted two yellow rubber rafts and the wreckage of the crashed blimp floating on the water. At 1558, Unimak took on board nine survivors and sank the unsalvageable blimp by collapsing the bag with 40-millimeter gunfire; the ship then landed the survivors at Portland Bight, Jamaica.
A few days later, on 12 July, Unimak joined with John D. Edwards (DD-216) in hunting for a submarine reported to be lurking nearby. Within a few days, word of a crashed plane sent the two ships speeding for the last reported position of an aircraft. Unimak located only wreckage and one body and buried it at sea on 16 July.
Unimak remained in the Caribbean through the autumn, tending patrol planes, conducting logistics support missions for advanced seaplane bases, and occasionally towing targets for the patrol planes training in the area. On 15 December, Rockaway (AVP-29) relieved Unimak, releasing her to steam north via Norfolk to Boston, Mass.
Arriving there at the end of December 1944, Unimak underwent availability at the Boston Navy Yard for the entire month of January 1945. She got underway for England on 14 February, but an engineering casualty forced the ship to return to Boston for a major propeller shaft alignment which lasted into March.
On 7 April, Unimak got underway for the British Isles and proceeded, via Bahia Praia in the Azores, to Bristol, on the first of two voyages to England to bring back supplies and men from decommissioned Navy patrol plane squadrons in the British Isles. On the second voyage, from 5 to 15 June, Unimak transported the men and materiel of Patrol Bomber Squadrons 103 and 105 from Bristol to Norfolk.
Departing Hampton Roads on 20 July, bound for the west coast, the ship transited the Panama Canal on the 26th and arrived at San Diego on 3 August. She got underway for Pearl Harbor on the 12th. The seaplane tender subsequently operated in the Hawaiian chain until 7 September when she headed for the Aleutians.
She operated in northern climes (calling at Adak, Kodiak, and Attu, Alaska; and once at Petropavlovsk Siberia) into November of 1945 before heading southward to prepare for inactivation. Subsequently reporting to Commander, 19th Fleet, in December, Unimak was decommissioned on 26 July 1946. She remained in reserve until transferred to the Coast Guard on 14 September 1948. She served the Coast Guard as Unimak (WAVP-379).