In real, no submarine commander has to worry about keeping the depth.The chief has to do that.So its useless to simulate, because you would never notice it. :hmm:
But, at 0 knots, if you "Hover" for a while, the boat sould fall slowly yes.
And you can not go "Fall" while on surface, because the boats ballast tanks are filled with air.And while attaking underwater, you have to be carefull.
Thats it.