And both characters went their own paths. And they have different powers. Don't try to pass off Namor's pathetic "somewhat copy the powers of an electric eel and a puffer fish (think it was that fish)" being something like Aquaman's telepathy was even before he learned to use it on more than marine life. And then of course Aquaman has never been able to fly. Then there's the fact that Namor is bullet proof while Aquaman's bones have been shown to be bullet proof but the bullets can penetrate his flesh and muscle. Then Namor is just slightly stronger out of the water, too.
Even with personality, with Aquaman getting more and more dark ever since the death of his 5 year old son in the 70s they still are different in that aspect of personality.
Now, archetypes in the loosest way of viewing them? Then fine. Both are aquatic super-empowered beings of royal origin. That's as far as their similarities truly go.
Though FYI Aquaman may have been created 2 years later but he had ties to Atlantis first.
