The Underwater World involves a large group of pedestrians dancing under the surface of the water in a circular formation, with animations similar to the ones seen in the mission, Life's a Beach.
This mystic place can typically be found at the outer edges of the State of San Andreas. Very few valid encounters were reported for this myth before a mod was created to simulate its effects, leading many players to consider its status to be unlikely. The fact that CJ can be seen in an above ground animation stance in the few screens-captures that exist of the supposed event also add serious doubt to its existence in an unmodified game, but there is a glitch, which can make those effects (you can even fly a cheated hydra using the glitch).
This myth could be a glitch.