I just loaded my world in the new 1.13_pre4 pre-release. I jumped in the pool I made, and when I got out, I was still in my swimming animation. I didn't see this bug in any other 1.13 snapshot before, so I think it is new in 1.13_pre4. I went to make a new world to test this, and it didn't load at all. It just said "Loading world". It had all of the default world settings (survival, no cheats, world type default, no starting chest, etc). When I went to close my game, it didn't close, so I had to go to task manager and force close it. I tried to reload the world I just made and the same thing happened, except this time, it also says "Loading world Preparing spawn area 22%". I tried to create another world with the same settings, and the same thing happened.