Extremely large, over several thousand block large lakes, almost "oceans" generate in single biome worlds with the new terrain generator. I think those lakes look quite cool, but I don't think they are supposed do be that large.
I found this while testing MC-192055. Then, I tried another world and I spawned next to a 1000 blocks long lake.

Steps to reproduce:

1. Create a new single biome world
2. Fly around, you may not always spawn next to it
3. Here it is.