Actor Tenoch Huerta revealed that he couldn't swim before joining the cast of Black Panther 2, despite the character being called the "Submariner".
Black Panther 2: Namor the Submariner Actor Couldn't Swim Before Taking the Role - IGN
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SPOILER ALERT! Could the oceanic earthquakes referenced by Nat and Okoye be the hint that introduces NAMOR the sub-mariner? Black Panther 2? - Quora
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Black Panther 2 actor couldn't swim before taking role as the Submariner
How Black Panther: Wakanda Forever's Water Scenes Came Together
Black Panther 2: Namor the Submariner Actor Couldn't Swim Before Taking the Role - IGN
Black Panther 2: Namor the Submariner Actor Couldn't Swim Before Taking the Role - IGN
Wakanda Forever: Namor actor didn't know how to swim