TMO and RFB are both great mods, you really can't go wrong with either of them. RSRD is also a great mod, it will make sure that the enemy is where they should be at the correct time. It also adds some ships to the game and i'm sure it took tons of work to create this mod.
The downside to RSRD is if you know your history then you know alot more than a real sub captain would. You can do a little reading and completely eliminate alot of the unknowns that a real captain would face.
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