Holding the jump key will lock the player's Y value when transitioning to and from the swimming mode

Is this the same issue that allows you to run/bounce across the surface of water? Like this:

Not necessarily. Your video is mostly showing that players are able to sprint in water without being completely submerged. This ticket is talking about what happens when a player is in the middle of transitioning to or from the swimming state (specifically, the point between going from a 2x1 hitbox to a 1x1 hitbox and back).

In your video, if you look at 0:14, you will see that you don't really fall or swim up; you float during the transition because you're pressing the jump button. That's what this ticket is talking about. I hope this helps.

So is the gameplay impact of this just that the player can avoid sinking when falling into water?

Yes, and the player can float briefly when leaving water entirely.

 
I can reproduce on Windows 10 when falling but the effect is not instant or dramatic. When I jumped into the ocean from Y=129, got the following results:

When you hold the jump key on land, you attempt to jump repeatedly. So, I would guess that each jump attempt just gives you a certain amount of upward momentum, whether on water or land. From my test results it looks like the Y-lock with the swimming transition occurs independent of the momentum giving by jumping. It think the Y-lock may be intended to make the transition seem smoother in first person. The bug would just be that these effects stack, or that may be intended too.

Thank you for your report!
However, this issue has been closed as Won't Fix.

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