Does a diver replace an underwater American flag at Pearl Harbor every three years? No, that's not true: There are no underwater flags at what is now Pearl Harbor National Memorial, according to their public information officer. The photo was taken in Florida at the Vandenberg wreck, a popular diving spot for tourists in Key West.
The claim (archived here) appeared in the Facebook group Facts you didn't know about on May 30, 2021. The text is:
Diver replacing the American Flag at Pearl Harbor - It happens every three years.
Lead Stories did a reverse-image search for the photo. The results showed the same photo the with same false caption that the flag was at Pearl Harbor instead of in Florida. But, one link from TripAdvisor.com had a different angle of the flag. It taken by a tourist who had visited Key West, and it was labeled correctly: "Photo: American flag on the Vandenberg wreck".
Speaking with a spokesperson from Pearl Harbor National Memorial also confirmed that the flag is not located at Pearl Harbor. Emily Pruett, Pearl Harbor's public information officer, responded to Lead Stories' email on June 1, 2021:
This a (sic) lovely image this does not occur in Pearl Harbor or at Pearl Harbor National Memorial. The two ships remaining from the December 7th attack, the USS Arizona and USS Utah are much too close to the water line and certain sections are visible above the water. The National Park Service does not maintain underwater flags.