Can't load a 1.12 water world in 1.13 snapshots

I began working on a map in 1.12 in a super flat world with the water preset. I tried to load the world in snapshot 18w08b but the game just says "Loading World." I waited 10+ minutes and it still said "Loading World" and tried to close the game. The game would not close when I clicked the "x" so I had to open task manager to close it down then getting the message that my game has crashed. I would love to use this world in 1.13 to use the new features.

Thanks!