The hidden map zoomed in. It reveals that the dotted line leads to a black dot, which has no island on it. However, this dot may be Rockhopper Island, but he did not draw it, or Penguins will find out the island. [citation needed]It's possible that if Herbert finds those islands, he will live there.
↑Rockhopper's maps are always a bit wonky, so I am not sure how accurate they are. However, he is quite the expert when it comes to sailing around the seas and exploring new places. Therefore, I think it's likely that you are correct about the islands - Swashbuckler's Trading Post, Dinosaur Island and Shipwreck Island.
