The bug:
The player stays on the water surface (doesn't sink) when holding the 'forward' and 'sprint' keys
How to reproduce:
Hold 'space' to stay on the water surface
Start swimming (walking) forwards while holding the 'sprint' key at the same time
Let go of the 'space' key
You are "walking" on the surface of the water without sinking (the "Y" axis doesn't change)
This is really hard to reproduce, you need to let go of the space key at exactly the right time. Might need a few tries.
This might be the same issue as MC-128013.