To duplicate: Be in survival mode, on a pc using a mouse and keyboard (I don't know if this glitch exists for Xbox or Xbox controller for pc). Find deep water. Aim the crosshair straight down. Press W. Instead of swimming straight down, like you used to be able to do (and still can in Java Minecraft), you will move forward laterally, only sinking very slowly to the bottom. The video clips show how it takes 30 seconds to slowly spiral down from a 61 elevation to the bottom at 47, mere 14 blocks. Activating sprint has no effect other than to make you move faster laterally. Activating sneak only seems to make you sink slower.

This is happening in both realms and worlds, only in survival, and is also happening to my friends who use mouse and keyboard. (I use a Logitech controller and keyboard emulator, but it's the same problem as using a keyboard and mouse.)

Because of size limitations, I've had to split the clip into two parts. Compare that slow death-spiral downward (I'm using a Respiration III helmet, or I'd be dead by the time I got back to the surface) in Win 10 Minecraft with the immediate and straight down descent that still exists in Java (and did in Win 10 before the 1.2.13 update).

Also, this is a duplicate of MCPE-32031, which was closed as "Unable to reproduce", probably because I wasn't specific enough in my instructions on how to recreate this problem.