• Do "EPEVehicle" diagEnable true in the debug console
• Drive any tracked amphibious vehicle into the water tapping left or right slightly so that the two tracks apply different force when getting into water
• Try going back ashore

I made a video: https://www.youtube.com/watch?v=Z-R_HKHxE8o&feature=youtu.be

This was tested with arma3diag.exe and latest dev branch (whatever version was the latest as of 2 Oct 2017).
Vehicles come from CUP mod but as far as i can tell its replicable with all of them.

As soon as you reach shallow waters the vehicle will spin out of control.

So my understanding of what happens is that when the vehicles gets into the water the game engine registers the force being applied by the tracks, and stores it until you make landfall again instead of resetting or following what you're doing when in water.
Normally you'd be going straight into the water, so the engine records that both left and right tracks are applying the same force pushing the vehicle forward.
When you drive back onto the shore the game remembers "hey, when this vehicle got into the water both tracks were applying the same force so im gonna push it forward regardless of whats actually making contact with the ground!".
Now let's say that when you were heading into the water you weren't driving straight for whatever reason, so the force applied by the two tracks wasn't the same.
When you drive back ashore the game now says "hey the right track was applying a much higher force than the left one when we got into the water, so im gonna do the same now!" therefore the right track will push much harder making the vehicle turn.
I could make the vehicle spin endlessly without even touching the controls (at 0:50 i wasnt even touching the keyboard or mouse), as soon as i was in shallow waters the vehicle would just go on and on.