Marshall Islands 1941: no ships...

In TMOwTw I was sent to the Marshall Islands in 1941 to patrol there. I was there on December 9 and patrolled there until December 22.
I've been shelled a few times from an island but haven't seen a single ship. I'm half way through my fuel oil and I'm about to get back to Hawaii now. Is this normal? During my patrol I changed course many times. Should I? Or should I have made my route exactly at the start of the mission so that the game could have presented an opponent somewhere along that route? I don't know... Can someone explain to me what's going on here?

Edit: And, off course, I saved my game many times...