Rearming in the Campaign?
 

When the torp stocks are low, how do you rearm?

Head to Holy Loch, Scotland (the big green square on the map) and you'll tie up automatically. Then hit "Re-arm and Repair" which will take you to a screen where you can load and unload torpedoes. Be warned though it seems that there's a bit of a bug there right now which (at least on the 68 campaign) isn't letting you do anything with your tubes in port so you'll need to do an XO report to actually load the tubes before your next fight.

Is there any other base to rearm?

Nope, Holy Loch is the only game in town.

which apparently you can lose by not sinking an enemy submarine fleet? I saw that warning in a mission statement. Luckily I was able to think it.

To bad no sub tenders.