Submarines shouldn't be able to stay submersed underwater for so long. There should be a timer which would act as air supply for the submarine, the timer itself should depend on the size of the submarine. With smaller subs having shorter times, and larger ones having larger times.
If the air supply runs out, then the pilot should start taking suffocation damage, and blindness.
This should be added to counter the immense damage subs do to ships and should also allow for more strategizing.
Good idea! I absolutely agree that submarines should have a limited air supply.
Maybe instead of nausea have the asphyxiating player experience nausea a few seconds before they are given blindness and (instant damage for one second at a time for 15 sec on, 15 off) Also i was thinking about nuclear submarines and hydrogen fuel cell submarines, which don't have to surface nearly so often. Also how would you how much air you had left? A GUI?