Since subs cannot fire at air units they are not as good as a transport in defens, not even a DD IMO.
As you said submariness used their periscope when they entered hostile waters to look for prey. That means in A&A terms that the movement was considerably reduced at that moment the submarine entered into a sea zone containing enemy ships, not before. So movment was 2. And don’t forgett about the submarines that were a part of a larger battle fleet. I will give you a short history of how the submarine was developed during the war.
The submarines in the Pacific
The interwar years, the submarine was little more than a coast defence weapon and woefully inadequate. The transformation in four short years was dramatic, with over 200 large fleet submarines in service in the US Navy by 1945. Although American boats often operated on the surface, later boats were cleared to dive to 450 feet and their hull strength stood them good when under attack.
Japan deployed squadrons of large submarines with the battle fleet and using midget submarines to attack the advancing US battle fleet. Some of these larger submarines were well designed to handle aircraft, but the concept itself proved to be misconceived. Some of these submarines carried aircraft.
The U-boats in the Atlantic
It was the aircraft that proved the great nuisance to surfaced U-boats. Even with radar, aircraft took while to develop the art of subamrine hunting, partly because Germans had a radio receiver that very effectively detected radar transmissions, enabling rapid avoiding action to be taken. By early 1943 shorter wavelengths confounded the German equipment and, for a space of three months, U-boat losses ran at an average of one per day.
Night-running on the surface, even to charge batteries, became prohibitively dangerous when aircraft received the 80-million candlepower Leigh Light, extra guns and improved depth charges with wich to mount sudden and unannounced onslaughts from the darkness.
The enemy survived bydeveloping the Schnorkel (snort), but slow, submerged transits, now occupied most boat’s endurance and even on station high surface speeds could not be used. Efficiency fell off rapidily.
Ultimately the Germans introduced the high-speed submarine in the Types XXI and XXIII. The former had a 16-kt submerged speed, under water control and advanced torpoes (the Zaunkönig acoustic torpedo, tuned to home in on the the enemys fast running propellers). These were the Super Submarines. Fortunately for the Allies these new boats was brought into service too late to have much impact.
