This could be two things:
1) You need to keep moving to maintain depth. However good your crew is, they can't balance out the boat just with the ballast tanks. You need water passing the dive planes.
Just 1 knot is enough to keep your boat somewhat stable.
2) There is a bug in GWX (and stock(?)) which means that if you order a crash dive and
then order a different depth before the boat gets down to the 75m(?) that the crash
dive ends at your boat will become less stable at low speeds.
Wait until the boat leavels off after a crash dive before ordering a new depth.
__________________
|