I have found pairs of tankers that were American in the carribean. The move pretty fast, and are sometimes armed, so the deck gun is out of the question.
I have seen a tanker with a merchant vessel as well, so there is occasions when you will get mini convoy's without escorts. It always pays to check the flag before opening fire. Did that once, and Uncle Karl wasn't that happy that I sank a German ore carrier!
At least I didn't get shot for it, and I still kept my command!