Where's My Black Flag?: Achievement STEAM of Assassin's Creed Origins

Captain Ships are marked by a red icon on the map . They are found only in water. When you zoom in one the symbol it says 'Restricted Area, Kill Captain 0/1'. You can also see the outlines of the ship when zooming all the way in. Usually these spawn in Lake Mareotis. Go to 8 of these symbols and kill everyone on the ship. Repeat a total of 8 times.