Worldborder can force player to spawn underwater

When the world border is set to a small area, the player sometimes spawns under the surface of the water. In some cases, the water can be deep enough to kill the player.

This bug is related to MC-7908, but is not quite the same. In MC-7908, the game is picking a bad spawn point. In this case, the game may have no choice but to spawn the player in the water. This bug can be fixed by spawning the player at the surface of the water, instead of on top of the gravel at the bottom of the ocean.