After creating a backup I opened up my world for the first time with 1.13. Many ocean chunks are missing and some chucks are switched with surrounding chunks. There are huge holes in the ocean. It allows me to see through the ground. It wouldn't let me walk into the missing areas. In some chunks you can clearly see particles and entities while others are completely blank. I tried shutting it down and reopening and it did the same thing as before. I tried opening the backup, but it also had the same glitch. The areas affected are all ocean biomes or next to ocean biomes. The other worlds I opened with 1.13 have no problems, even the one that is a copy of the world with the bug. This world is from 2016. it was created before 1.10 came out, but after 1.9.

Map download: https://drive.google.com/file/d/1YB8pYxS0mdCQgmfhO8c5OGQpFLQWfHhW/view?usp=sharing