Battle surface and reaction time.
 

I would like to see the dive planes controlled separately from the depth guage. This way you could blow all tanks and hold the sub down with the planes till the tanks were fully blown then release them and pop up on the surface more quickly (battle surface). It would also be nice if you achieved surprise that the enemy would have to sound battle stations and have some time to man the guns before being able to engage you. Now as soon as the target spots you they open up. It takes them 0 seconds to be at their battle stations.