You may have hit a subnet or mine (Probably a mine). Both were present in the German owned harbors for protection. If you go to your nav map, in the top left corner of the map are a bunch of pull down sheets. One is labled "Oberkommando Der Kreiglmarine Minen- und Abwehrnetzakarte." in the lower left hand corner of the sheet.
This page has on it the maps of all the German ports and shows the locations of the mines and subnets at the mouths of the harbors.
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