There's also a way to make sure you get the name and number of the sub you want when starting a new career, so you won't have to go through the save-game-then-change-files approach noted above. I've created a small JSGME-ready mod that I use to get the one I want (USS Pantherfish, SS-55 at the moment) - you can D/L it here:
http://files.filefront.com/Pantherfi.../fileinfo.html
Just check inside the files (you can open/edit them with Notepad) for the "USS Pantherfish" and "SS-55" references and change them to whatever name and number you want, install/activate your tweaked version of the mod via JSGME and start a career - you'll automatically be assigned the name and number you want right from the start of your career. You can always change it again next time if you want a different name/number, or simple de-activate it via JSGME if you want to go back to having the game assign them normally.
Enjoy!