The bug
When colliding with water, the game doesn't properly calculate an adjusted falling speed (really you shouldn't fall at all if you haven't loaded in yet, but I think this is to prevent you on servers from getting Kicked for Flying), which means, depending on how quickly the game loads the chunk, you will be at different heights once it loads. If it takes less time to load, the game will have you higher up (since you fall more slowly in water), but if it takes longer, you will fall quicker because the game doesn't know the water is there and just treats it like air.

This is usually a non-issue for teleporting, but in the case of custom maps where you are teleporting long distances and you want the player teleported to a certain height in water, the height will be substantially changed depending on how fast the chunk loads.

This also causes players when logging in to quickly fall through water.

Steps to reproduce

Preferred testing method

1. Log out of a world at the surface of a deep water area. You can't be in spawn chunks.
2. Log back in
3. You will have fallen significantly down when you log back in. In my testing, I fell about ~13-14 blocks while logging in even though it only took a few seconds to log in, and you don't fall that quickly through water. (As a rough estimate, it takes about 30 seconds to fall that far through the water, despite the world not taking nearly that long to load) Once I even fell ~22 blocks