Drew McWeeny of HitFix (who has a history of quality and reliable reports) is reporting from multiple sources that Jason Momoa will in fact play Aquaman in Batman v Superman: Dawn of Justice, as has been rumored for several months. They note that while Warner Bros has yet to comment officially, they have even confirmed that character designs are being finalized by Zack Snyder.
The character’s involvement in the film will not be much more than a cameo, with Arthur of Atlantis upset over the World Engine in Man of Steel and what it did to the more watery areas of the world. The report says that the contract will (naturally) also include Justice League, which in turn is rumored for May 2017.
Momoa has both kingly and fantasy experience, of course, playing the barbarian leader Khal Drogo in Game of Thrones, as well as playing the barbarian in Conan. He has been rumored for other comic book movie roles, including this one.