Subs are slow, but their stealth and power are never to be underestimated.
For those who are attempting to sink a submarine, don't use torpedoes. Torpedoes are too shallow to strike at a submerged submarine. Only depth charges are effective against them.
Destroyer-launched depth charges are effective against submerged subs, but air-dropped depth charges can in fact hit surfaced subs too. Dropping them like a bomb on a surfaced sub causes, in fact, more damage than any kind of bomb or torpedo. A single pair of air-dropped charges is more than enough to sink a completely healthy sub.
Most kinds of bombers can equip depth charges (most notable dive and torpedo bombers) but are the deadliest when equipped on recon planes like the PBY Catalina. Their innate sonar allows them to detect submarines on their own and strike without fear of counter-attack, while dive and torpedo bombers rely on another ship's sonar to identify the target.
Another victim. Heavy cruiser Mikuma is sinking.
Note that subs require a lot of micromanagement. In the event you're playing a map like Coral Sea, it's best to give your other units away and focus on the submarine until it goes down, then ask for your units back.
It's also a good idea to have a fighter patrol the skies above you. Not only will this protect you from Depth Charge Judies, but they will also allow you to see them come in. As a submarine you will not see any planes on radar unless you surface.
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