Unfortunately, SH and pretty much every other sub sim is not totally realistic in this area, as it would probably require a supercomputer running all sorts of chaos algorithms to model it at least well, if not truly realistically. So the reduced buoyancy in the sim is about the best simple comprimise.
As seafarer notes, if you ever bring a boat to a halt out on the open ocean in real life, the amount of drift you get on currents is absolutely frightening, and you can quite easily drift a mile or more in ten minutes near estuary outflows, which extend fo many miles. So in theory your sub shouldn't even be able to remain laterally stationary either, as it would require thrusters to pull that off. And that drift would put you in water with altering densities, as the temperature, salinity, etc fluctuated.
Real U-Boats, and modern subs for that matter, will drive dynamically to the depth they want, and then try to maintain that depth by altering trim, but some fluid movement over the planes is the simpler and more desirable method, and the problem gets even worse if you launch a torpedo and suddenly lose all the weight of it as it leaves your sub, subs being equipped with compensating tanks which flood to aleviate the problem a little.
This is why most WW2 subs would perform a daily 'trim dive', to give the chief an idea of what was a likely setting required for maintaining position at various depths given the sub's altered weight from food consumed, oil and lubricants used, ammo spent etc, all of these changes alter the calculation. Even the sub itself gets smaller as it dives deep and is compressed by the water pressure!
I was reading the other day that when the
Pittsburgh was launching Tomahawk cruise missiles in the Gulf War, even though in theory it can launch them in a matter of minutes, the skipper actually started the countdown to launch twelve hours beforehand, and elected to slowly approach the launch point from the start of that countdown, so that he would be able to ensure he was at the correct point and depth at the right time, and that's a modern nuclear sub with computers assisting the trim calculations!
So if it's a bit tricky to maintain depth in the sim, that's definitely realistic, even if it is simplified.
Chock