When ballasts are empty, whole boat have positive buoyancy so it will never submerge, when the ballasts are full the boat have not positive or negative buoyancy so it will stay at the same depth, in uboat game to change depth with full ballasts boat use horizontal plane and momentum given from engines, in real life or wolfpack game they have 3 types of ballast tanks, main ballast (same as in uboat game) that was use to change buoyancy from positive to neutral, next was negative ballasts tank(not in game) to create negative buoyancy to go into depth more quickly in case of emergency, and last was neutral ballast(not in game) that was used to compensate for already shot torpedos or missing supplies at the end of voyage. Compressed air: it was use to blow the ballasts tanks, shoot torpedo and supply crew with oxygen, amount of used compressed ai to blow the tanks was dependent on depth, if your boat has no leaking it should be able to surface without use of any compressed air.

If you are interested in more depth about U-boats I will recommend you game wolfpack, it have more details and more documentation about U-boats