In the midst of generating world after world (to find a jungle) in creative; I happened to spawn in a few with these very odd square oceans as well as one with an odd elevated chunk. The first of which was rather large ocean or lake that was entirely square, ignoring elevation, it just plopped itself in the middle of a few biomes. I flew around a bit and the ocean continued its square nonsense. I attempted to replicate it with its seed "scumbag" (let's just say I was bored and wanted a jungle), however the odd ocean was gone and the world generation was fine. The second world was similar, only it had a smaller ocean. And the third appeared to have a single elevated chunk. (pictures below: the first two of "scumbag", the third of the second world, and the fourth picture of the funny chunk.)