Board Thread:Naruto Discussions/@comment-26636534-20160527151459/@comment-26636534-20160528104218

Thekillman wrote: The world map is highly stylized, it shows no lakes (even though the Valley of the End itself has a lake). It shows no rivers either.

So yea, there simply is a big sea outside that isn't marked on the map.

Except, Amekagure is land-locke, it is far from any great mass of water. Even if there was a lake, no lake has water pressure that is high enough for the bottom to be unreachable.