So discovered this when converting my world, The water you see past the border on a 360 world extends for 1104 blocks before it stops, the entire world itself is only 864 blocks wide and only takes up 67%~ of the center 4 regions. New generation will only start at 1536 blocks when a new region and chunks are generated. The water is only 15 blocks deep or so, the rest is complete stone until bedrock, If playing after caves and cliffs, generation below 0 will spawn in as normal. This is also the same for the nether, but behind the bedrock walls is only the void till 1536 blocks.