If you want to have a career of over 3 million tons of which I have achieved. Keep doing your mission to the minimu, or just forget it, and go into tokyo bay and hit the first port on the left. During the first 3 years there are two heavy crusisers, and mid war carriers, and destroyers = cruisers. I have hit these and keep coming home with 80-120,000 Tons plush the convoys that keep coming in. Also hide in this port against the east wall at persicope depth. The destroyers always are on the other side, and will never harm you, if you have the unfortunate encounter with them.
Keep surfacing near the moored seaplane and throw a few deck cannons at them and dive again. After a while they all catch fire or hit each other and sink.
Also if you wanna risk it as I always do, go into Kobe, and take the long channel until the islands, and sometimes sit for a while. But when you get there ALL during the war, There is 2 cruisers, 2-3 Carriers, and 2-3 Battleships, and coming home from there with well hit shots will earn you nearly or Over 200, OOO Tons...
Keep it up Boys
God speed, and good hunting, and if you need tips on where to hit vessels with a one shot kill for cruisers, and a two shot kill for Carriers, let me know.