The Arrowverse has introduced and teased various Justice League characters, with Aquaman being one of them, through a handful of Easter eggs over the last couple of seasons. While The CW hasn’t always had clearance to introduce the bigger players, the rules seem to be evolving season after season. From the latest tease of Diggle becoming Green Lantern and the upcoming Superman and Lois series, to the addition of the Bat-mythology on Batwoman, the larger Justice League line-up is becoming part of The CW’s Earth-Prime. However, a few characters have yet to either have their statuses specifically declared or even get referenced directly by name. Batman has been established as having been missing in the Arrowverse for a few years, which allowed Kate Kane to become Gotham City’s new protector.
Batwoman has also confirmed that a version of Wonder Woman is out there while Legends of Tomorrow revealed that Themyscira existed on Earth-1. However, another big Justice League member that has been teased for years is Arthur Curry a.k.a. the king of Atlantis. But unlike Bruce Wayne and Diana Prince, Aquaman has mostly been teased through the existence of Atlantis that began in The Flash season 2. With The CW shows consistently referencing Atlantis, the question that becomes more relevant is when viewers will actually see Aquaman joining the Arrowverse. Despite not being referenced by name, Arthur was alluded to as early as in the pilot for The Flash through a deleted scene when Joe West references Barry visiting the Amnesty Bay to investigate a story about a man who could talk to fish.
A deleted scene from the sixth episode of Legends of Tomorrow season 5 mockingly referred to Arthur when talking about the concept of a superhero being able to breathe underwater and talking to fishes. Clearly the idea of Aquaman seems to exist within their world, but what's unclear is if Arthur is known to them as a real superhero or if it’s a similar situation to Black Lightning before Crisis on Infinite Earths. Pre-crossover, characters like Superman were known on Jefferson Pierce’s Earth as fictional superheroes. That was why when Jefferson was stunned when seeing two Men of Steel after first meeting the heroes of Earth-1. It’s possible that if Aquaman ever shows up in future seasons that he is only a comic book character on Earth-Prime.
But given all the callouts to a man being able to talk to fish as well as the status of Atlantis, it seems unlikely Arthur would be limited to being a work of fiction. With Jason Mamoa currently portraying the king of Atlantis in the DCEU, it might take longer for The CW to be able to introduce Arrowverse's Aquaman. However, the restrictions are always seemingly evolving as The CW is gaining more access to the DC sandbox. Grant Gustin and Ezra Miller have been playing The Flash at the same time since the series began in 2014. Superman has gone from guest appearances on Supergirl to now getting his own standalone series next season. Black Canary is and has been featured both on the small-screen through Arrow as well as this year’s Birds of Prey film.
The idea that Aquaman could exist on TV while Mamoa’s franchise is still going might not be as impossible as to before. With Crisis on Infinite Earths rebooting the Arrowverse and establishing the foundation of their Justice League, Arthur could make his splashing on-screen arrival sooner rather than later. What is the likeliest scenario is that a visit to Atlantis, on-screen, is when viewers will first meet Aquaman whether it's in a crossover or through a storyline on one of the shows. Visually, it may be tricky to show viewers Atlantis which would require a lot of heavy and expensive VFX work. What's also possibly preventing the Arrowverse is the financial costs that would come with shooting underwater sequences. While Aquaman would likely not get an Arrowverse spinoff, seeing him every once in a while would be a solid pay-off to all those teases even if it doesn't always come with stunning water scenes.