I realize this is an old thread, but just wanted to clarify something on the original post.@Tobit:
I was looking through the 1941 rulebook last night, and noticed that there is a provision for transports that they cannot disgorge units while ignoring enemy subs in the sea zone they are dropping off from, unless they are accompanied by a warship. So even though the SZ could be treated as friendly under normal conditions, in this case it can’t. Is this a new rule? I don’t remember it from 1942.
I believe this is only true if you are performing an amphibious assault. If you are offloading in a friendly territory (moving in non-combat), then you can ignore enemy subs.