Just to add one thing to this thread: all of the boats belong to a particular upgrade class. When you are offered a new boat, it will be from a higher class number; conversely, if you are downgraded, you will receive a lower class boat.
- Class 0: S-18 and S-42
- Class 1: Porpoise, Salmon, Sargo, Tambor, Gar
- Class 2: Gato
- Class 3: Balao
And that makes logical sense. S boats were an entirely different design than fleet subs. Porpoise-Gar were all pre-war fleet subs. Gatos were being commissioned just as the war was about to begin, and Balaos started showing up in 1943.