The Ocean Portal is a myth in Grand Theft Auto III and Grand Theft Auto: San Andreas. The myth is about, when a player swims really deep into the ocean, they don't drown, instead, they go through the sea floor, and suddenly get teleported to a different area of the map. The myth originated from the Bermuda Triangle myth investigations, and attempts from players to reach Liberty City.
Grand Theft Auto III
The Water Teleporter appears to be a non-solid place in the water. If you fall in it, it will take you to a random place nearby, akin to the Back O' Beyond Teleporter. The reason for this is that the only solid water is in the boundaries of the map. The rest is out of the boundaries and is non-solid.
This is known to be a glitch.
Grand Theft Auto: San Andreas
The player, who started the myth was actually just trying to fly to Liberty City. He never made it, because his plane crash landed into the ocean. However, instead of drowning, his airplane sunk to the bottom of the ocean, went through the sea floor, and then he was teleported back to San Andreas. When myth hunter Mike94Chuck investigated, he also was teleported back to San Andreas.
If the player flies far enough out into the ocean, the ocean floor becomes unsolid, so the player can sink through it. However, the space beneath the unsolid sea floor is Blue Hell, so the sea floor acts as a large entrance to Blue Hell. When the player enters the Blue Hell area, he gets teleported to a random area of the map. This is a game mechanic that prevents players from falling through Blue Hell indefinitely, if they ever enter it by accident.