It is the only UFO that can be found without attaining 100% completion of the game, as it is not a functional one to view in the sky (which needs 100% completion to be seen).
The Sunken UFO Wreckage can be found off of the northern coast of San Andreas, just north of Procopio Beach and a few yards east of the large peninsula. The UFO appears to have crash-landed into the ocean and sunken to the ocean floor. Although largely intact, the UFO shows some signs of damage, most likely the result of the initial impact of the crash. Some weeds and underwater debris have collected on the top of the UFO, hinting that it has been there for a long time.
The UFO is possibly referenced by Omega, who claims that a UFO has crashed recently during his random encounter. The Sunken UFO could also possibly be the source of the Spaceship Parts scattered around San Andreas. After the mission Three's Company, there is a Weazel News segment on the radio in which a hiker (Omega) mentions seeing a UFO crash into the ocean.