I've sunk several ships and gotten grey or no mark whatsoever on the map.. this not only in shallow areas but pretty much anywhere. A lot of times, the map makes no mark when I have the periscope down, I'm submerged, and I don't actually "SEE" the wreck when the crew says that it's sinking. I will usually get no applause or the "YAY !!! SHE'S SINKING" from the crew. Just the announcement at the bottom of the screen. Even though it's not marked on the map, I've been getting credit for the kills.
Hope this puts you a little at ease.
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