[XXi]

Depends on the time. I`m careful before the Invasion of Norway as there are quite many Norvegian vessels. Same for Greece and other countries.
After December 1941 I recognize that at least 95% of the vessels there are enemy, so I don`t care any longer, no matter if it `s a lone merchant or in convoy. One of few exceptions is the Brazillian coast - I avoid attacking their ships before the country enters the war.

Now, I play a D2 campaign and in the Indian Ocean - Cape of Good Hope area there can be, actually, only enemy units.