In GWX it is in fact the opposite. The buoyancy model is made to let your boat rise a bit from the ordered depth if speed is too slow. Both alternatives are realistic, in fact. Buoyancy is very difficult to keep right, an U-boat in the water is like a balloon in the air, a very slight change in wieight causes it to slowly raise or sink, the balance is very very difficult. In a submerged object it is not as bad, but then again there are factors like water salinity, temperature, etc. that will make it harder.
Thus, U-Boats went under water with either positive or negative buoyancy and were kept at level thanks to the diving planes. No speed, or not enough speed= not enough lift/downdrag from the dive planes = you can't keep steady depth.
Wether the boat had negative or positive buoyancy, depended on the situation. Hence, both NYGM and GWX are realistic.
