I believe the subs use hit points like the merchants. Although bulkheads showing PATCHED, never truly like new, diving is a craps shoot. It is a chance you will have to take as most Commanders had to take. No one can truly say how deep the sub can go after the hull is damaged. It is a wild guess, even for the engineers. Sometimes it just best to duke it out on the surface while hauling butt to port for a refit.
Again, gents check to see if the box above the damaged area is blue. If so you have to wait until it is pumped out. Even 1 ton of water will screw with the submarine.
Case and point, USS Trigger fired all forward torpedoes going after a carrier. As you know water and the air used to push torpedoes out of the tubes is sucked back in to keep the location of the sub under cover. Well, with over 2 tons of water in the forward tubes the USS Trigger was in a uncontrolled dive to 450 feet until she could spool up the electric motors to keep her from going deeper. Mind you at this depth the packing around the propellor shafts will leak so she was taking on more water. She was able to keep nose up and steady depth with the electric motors to have the pumps dispel the water. She was able to surface.