In the latest addition of the Official Playstation Magazine, Australia, there is mention of several new Injustice characters that have not been confirmed yet. One of them that the magazine does confirm though, is Aquaman! The article mentions and explores the likely possibility of Shazam, Doomsday, Sinestro, Ares and Black Adam in the game. Ed Boon has also been making subtle hints at pretty much all of these characters and thanks to the Official Playstation Magazine, Australian edition... Aquaman is confirmed!
The article confirms Aqauman, by the following: "Let's address the 6'1", 325lb fish-man-shaped-elephant in the room straight away. Aquaman is going to star in the next fighting game being made by Mortal Kombat creators, NetherRealm.."
If you wish to check out the article, be sure to pick up the Official Playstation Magazine at your local retailer!
UPDATE: A recent tweet by Geoff Johns of DC Comics (thanks to the help of @JaeRoar ) confirms that Aquaman is playable! Thanks to MKO for spotting this!
Credit: Injusticefan14, pherleece
Thanks to @pherleece for the tweet heads up