Well, again, two items;
1. Successful career depends on renown. Near as I can tell the game actually pays no attention to mission completions, only the total renown earned however you earn it. Unless you're playing a supermod that increases renown for completing mission objectives and decreases renown for tonnage, there's really no point in trying to complete objectives. I have gone through an entire successful career without ever - not even once - completing a mission objective, I deliberately ignored the objective for every patrol, even going the opposite direction. "Patrol Sulu Sea", okay, so stay out of there, run up to Osaka and blast everything in sight instead, here's your victory medal, want a new sub? No reason to go into shallow water if you ignore objectives, pick your own patrol area. Not realistic, but that's how the game is programmed.
2. You really WANT to complete objectives, go to the shallow water star, wait the allotted time, head back for deep water avoiding contact with anything in the shallows. Even if it says "sink enemy merchant shipping in Java sea" only the first part counts, for the mission complete checkmark all you need to do is sink 10 thousand tons of merchies (also unrealistic, in real life they wouldn't specify merchants and would jump for joy if you sank nothing but warships instead) and the program doesn't care WHERE you sank them, once you hit 10 thousand tons here's your mission complete renown - plus the tonnage renown, which is actually more important for success.
Obviously everyone should play however they want, but it IS a game, it IS for entertainment, if it gets frustrating and no fun then play differently.
