Marvel Studios president Kevin Feige has reiterated that, while he'd love to have Namor join the Marvel Cinematic Universe, the deal is still oh, so very complicated.
This stems from designs Universal Pictures had for the character in the past, including a failed movie with Legendary Studios. Despite Universal's grip on the character expiring in 2006, the MCU Avengers-verse in full-swing and fans often clamoring for Marvel Studios to bring the Sub-Mariner into play (there was even an Atlantis easter egg in Iron Man 2), Feige maintains that bringing the ruler of Atlantis to the big screen remains a lot more difficult than it sounds.
When asked by IGN if it's still a tricky situation, he replied, "It is. I think there’s a way to probably figure it out but it does have -- it’s not as a clean or clear as the majority of the other characters."
Namor's distribution rights were sold to Universal when Marvel was facing bankruptcy in the '90s, as Feige said back in 2014. The Namor contracts still involve “other parties" -- which likely means producers or production shingles had signed on at various points between 1997 and 2006, thus complicating proceedings.
“Let’s put it this way -- there are entanglements that make it less easy,” Feige said. “There are older contracts that still involve other parties that mean we need to work things out before we move forward on it. As opposed to an Iron Man or any of the Avengers or any of the other Marvel characters where we could just put them in.”
Ironically, Marvel Studios worked out a deal for the Hulk with Universal in the past, although it doesn't include the rights to his solo outings -- which actor Mark Ruffalo even criticized, stating that Universal just isn't willing to "play nice." Screenwriters Chris Markus and Steve McFeely, who helped engineer the Russo brothers' vision for the MCU leading up to Infinity War, have even indicated that if it ever became possible, they'd love a crack at the Atlantean.
But as it stands, Marvel fans will just have to accept for now that the DCEU have beaten them to an underwater adventure featuring the sea king of Atlantis, courtesy of James Wan's Aquaman in December. Given that Marvel worked out a deal for Spider-Man with Sony, and the Disney/Fox merger could bring the X-Men and Fantastic Four over, undoubtedly fans will still hold out hope that Namor will swim into the MCU someday.