This is a term used for the ships permanently assigned to Pearl Harbor and the Third Fleet. The term comes about for an obvious reason, pineapples are grown in Hawaii, so sailors used some common sense for that one.

The fleet was formed in June 1943 under Admiral Halsey. The fleet has always been station at Pearl Harbor since it was formed. During World War II the fleet operated Solomon Islands, the Philippines, Formosa, Okinawa and the Ryukyu Islands. The flagship of the fleet was USS Missouri.

Shortly after the war the fleet was removed from the active list and placed in reserve.
In 1973 the Third Fleet was recommissioned as an active command. The Flag was transferred from the headquarters building to the USS Colorado AGF-11. The Colorado was transferred to San Diego in 1991, so along with her went the command of the Third Fleet. In 2003 the flag was transferred from the Colorado to a shore building at Point Loma, San Diego, California.

There are currently 4 Nimitz class carriers assigned to the Third Fleet. Third Fleet's primary mission is one of conflict deterrence, but in the event of general war, it would conduct prompt and sustained combat operations at sea to carry out the U.S. Pacific Fleet strategy in the theater. Such operations would be executed well forward and early in a conflict to carry out the primary wartime mission of Third Fleet, the defense of the western sea approaches to the United States, including Alaska and the Aleutian Islands.

So technically it is no longer the Pineapple Fleet. But, the name is still used when talking about ships stationed at Pearl Harbor