There is a slight move upwards when the player "sprints" underwater. (sprint meaning pressing the sprint key while underwater.) Very small thing but also very noticeable.
- Legacy ID
Go diving underwater (preferably with a diving suit), keep swimming at your normal speed by pressing W only. Then press Shift to "sprint". You will easily notice the animation throws the player a little bit upwars while "sprinting".
Do it near a flat sea-bottom to use it as a reference.