There were recorded cases of a depth-charge going of right above the boat forcing it down 50-100 feet, and at least one case of a depth-charge right under the boat forcing it to the surface. None of this is modelled in the game, of course.
A boat being forced down or up a little should happen all the time, and at any speed. Of course when moving it's easier to use the planes to adjust the depth. While it's extremely difficult to keep the boat in trim when not moving, at 2 knots it should be controllable, so you shouldn't automatically sink at that speed.
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