It is an issue on the Europe side, where one sub can affect the income of two separate powers boardering the same sea zone.
You can actually do that in Pac, I think. Canadia is Brit money, no?
if one sub attacks another sub, can the defending sub submerge before being shot at?
Yes indeed.
When the attacking sub make his combat-move, the defending sub may choose to submerge or fight.
…unless there is an attacking destroyer too
Really? I had no idea. How does that make sense? If one sub submerges to flee, the other could also submerge to pursue it. Is this true that subs can submerge against other subs?
yes. they can disappear in to the dark deep sea and only the almighty destroyer can find them.