Not sure about this part...but I had that happen before too in shallow water. Every time I sink a ship I go to the Radioman and send a report. I get a report back stating "Good work! Blah, blah, blah". When that happens my credit appears on the Captains log. Funny thing is that the sunken ship icon does not appear (shallow waters), but I have the kill and receive the renown when the patrol is over. This has happened to me twice. I am not saying sending the report will fix it, but for me, it seems to work. Might be a dilly of a coincidence, but twice under the same circumstances might make it a likely "fix".
Now another thing too is that I also used my deck gun and shot the flag off on one of those occasions. Then the red icon appeared on the map. I see this out in deep water where the icon will appear before the water is over the gunwales, so not sure if the shallow water situation is an issue. Anyway...I haven't run into the problem of no credit for kills since I report every sinking just after it happens. Just something I noticed....
