1. Go into spectator and/or creative mode.
2. Try sprinting in the air or water to fly faster (this works fine).
3. Try dipping in or out of the water, crossing the threshold (this takes you out of the sprint mode).

4. Staying half in water and half out of the water makes you unable to sprint around faster at all in spectator / creative mode.

Discovered this while flying around underground, and water caves kept giving me a hard time.