User blog comment:Ghmorello/Chapter 921: Rock The Dragon/@comment-2602:304:68C1:BBC0:F454:9167:9804:8F6C-20181019125450/@comment-24850639-20181019154413

After being shown someone like Dellinger from the Donquixote Family, you were supposed to make the connection that Jack was at least part fishman when Oda revealed his mouth. He also talked underwater, which was a clear indication that he was capable of breathing.