This glitch has happened twice in the last three days. I start a marketplace world that I own (How To Train Your Dragon and Emerald Tycoon in particular), but when I leave and come back a few days later, the world is replaced by a giant ocean, with nothing else in the world. I have deleted both examples of this glitch, so I don't have any screen shots. Both worlds were working fine previously.
Update: I attempted to create a new version of Emerald Tycoon, but it was just a giant ocean as well.