For the German side, Type II, Type VIIC, and Type XXI U-boats were available. For the US, your choice is between S-class, Gato class, and Tench class, with the mission determining the actual type used. All subs had reasonably correct specifications. S ub Battle had an impressive list of features. The year the mission takes place determines the level of equipment available.
Aircraft are present.
If the sub is near the shore or a carrier, occasional patrol aircraft could cause a bad day at sea. Ships in an attacked convoy will change speeds and courses in an attempt to escape destruction. Damage a ship and its speed diminishes. Radio is used to give reports and orders, including downed pilot locations for pickup. Missions also include shore parties and minelaying.