Namor is essentially the Aquaman of Marvel; both are half-human and half-Atlantean, both are the King of Atlantis in their respective universes, both are extremely powerful characters who can fight some of the most powerful heroes of their universes such as Superman and the Hulk and yes, both can talk to fish and other marine life.

What's interesting is how Aquaman is so much more relevant in modern pop culture than Namor is. Namor has been very influential on superhero comics. He first appeared in the Golden Age of comics all the way back when Marvel Comics was still known as Timely Comics and he was created in 1939, 2 years before Aquaman appeared in 1941 and just a year after Superman made his debut at DC Comics in 1938. He was the first superhero who could fly -- back in those days, Superman was only able to leap over tall buildings (hence the whole "able to leap tall buildings in a single bound" tagline) and wouldn't actually fly until a few years later. He was even retconned into being the first mutant in the Marvel Universe and forming the Illuminati alongside Mr, Fantastic, Iron Man, Professor X, Dr. Strange, and Black Bolt.

Somehow though, Namor still remains mostly unknown to the general public. This is probably because Aquaman has had more exposure outside of comics such as The Superman/Aquaman Hour of Adventure in or Super Friends. DC has treated Aquaman much better than Marvel has Namor by featuring him in media outside of comics such as the new Justice League movie or Teen Titans Go! while also trying to keep the character relevant in comics. Namor on the other hand is much more of a side character and almost always appears in comics where he is not the central character.