The bug

The player is able to stand up when swimming through a one-block high gap, the player can stand up even though there is a block above.

The name tag will visually appear lower than usually and in the first-person view it still looks like the player was still swimming, but when in third-person mode (or as seen by another player) you see that the player is actually standing. If you sneak, the first-person view will jump up inside the block above.

If the block above the player is solid (in the example: the red glass block), the player will suffocate when sneaking (if in survival mode) and won't be able to leave the block it is in without destroying any blocks.

To reproduce

1. Build this:
   
2. Go into the water and start swimming
3. Swim into the one-block high gap
4. Touch the ground or press Space without pressing Ctrl
5. Sneak

Video

https://1drv.ms/v/s!AjMAPYK7UIdcsDaGbKsYiS1m89Rk