The latest episode of DC Universe's live-action Doom Patrol series has revealed that Aquaman is a publicly-known member of the Justice League. While the King of the Seven Seas doesn't appear on-screen, the fact that he is real and that he is known to the world at large are firmly established.
The shared reality of Doom Patrol and Titans has been given a surprising amount of definition in a short amount of time. While the exact details of the history are maddeningly vague, both series have done a good job of suggesting a larger world outside the context of the individual shows. Easter Eggs, such as magazine covers discussing Wonder Woman or characters wearing T-shirts with Superman's distinctive logo help to define the larger setting. The dialogue of both shows has also included a number of off-hand mentions of characters from the classic DC comic books that are not seen on-screen, such as The Flash, and now Aquaman.
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"Puppet Patrol" - episode 3 of Doom Patrol season 1 - sees the Aquaman name drop come through a sarcastic retort by Robotman. The action of the episode sees the team taking an ill-advised road trip to Paraguay in search of the kidnapped Dr. Niles Caulder. The stresses of a family vacation only add to the preexisting tensions between the various team members. Perhaps the most rocky relationship is that between Victor "Cyborg" Stone and Cliff "Robotman" Steele, who seems to resent the young superhero for both his take-charge attitude and the fact that he has similar powers to Cliff while looking more human.
Cliff vents his frustration with Cyborg's leadership with several references to other, more famous superheroes, who Cliff says could do a better job running things. At one point, he sarcastically makes reference to Batman while insulting Cyborg's ability to rally the team in a moment of crisis. The peak of these insults comes when Cyborg almost loses his temper while Elasti-Woman is pulling herself together in their hotel bathroom and everyone else is waiting and anxious to get back on the road. In the middle of all the arguing, the keys to the Doom Patrol's bus go missing from where Cyborg put them down. This causes Cliff to declare, in mock disappointment like a disapproving father, "That's real irresponsible, Vic. I'll bet Aquaman never loses his keys."
We'll ignore the fact that Aquaman probably doesn't have keys to lose, unless we're referring to the Superfriends' version, who owned a jet ski for some reason. The fact remains that this line has confirmed that another member of the Justice League is out there somewhere in the world of Doom Patrol. All that is needed for the classic team to be complete now is confirmation that Green Lantern and Martian Manhunter exist.