I was playing with friends, and my friend tried to glitch through a glass pane (it didn't work) and he accidentally swam a bit lower than the block. He then started swimming, in the middle of thin air.

Steps to Reproduce:
1. Make roof, out of any block, and then create a water source block in a glass pane above the roof
2. Swim in the water source block, and swim slightly down, below the block
3. You're now fly swimming!

Observed Results:
It makes you swim, out of water

Here's the link to the video: https://youtu.be/ybDTyV2awSI