That's cool and all, except comic book movies aren't directly based on Greek Mythology, but the comic books (which yes, can be based on Greek Mythology, but they aren't the same), and if Aquaman is blonde in the comic books then that's the accurate way to portray him as that is the source material. Furthermore, even with your logic Momoa was a poor choice since he doesn't look Greek at all.
Many cultures have had a big connection with the sea. Point is, it's not something that only fits polynesian people. Aquaman being white/blonde wouldn't have taken anything away from the fact that he's an ocean based superhero.
Aquaman didn't need a new look. His "bad reputation" was blown out of proportion because people only remembered the silly Super Friends version, in which by the way, everyone was silly. There was actually nothing inherently wrong about the character. In fact the Aquaman from the comics was blonde and still was a badass:
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Problem is most people ignore those versions and still remember Aquaman as the useless dude who talks to fish. If you look at Momoa's AM, he still talks to the fish and he still has the classic suit. So ironically, they pretty much kept everything that supposedly made Aquaman a joke. So the change of looks was unnecessary.
Like I said, Momoa worked mainly because he was playing himself, but that doesn't mean another actor couldn't have worked too.
Polynesian skin is slightly more darker from that of the proper Mediterranean people, Atlanteans are basically the descendants of the ancient Greeks. Ancient Greeks themselves descended from the middle east. Therefore Momoa is a good choice from that point. After all he's half polynesian.
The Fact that in the comics Aquaman has blonde hair, it does not mean it has to be so. Most movie goers don't read the comics and don't know that AM is a badass, for many he WAS the silly character of super friends Version. Therefore a change was BADLY needed.
Besides, If one has to stick to the comics, any aquaman sequel movie is screwed before its even filmed. One of the main problems with the Aquaman comics, is that it contradicts itself at every turn. First you have Arthur who wants to be Atlantean, than he tires and wants to live on the surface with Mera, yet he goes back to Atlantis to be king once again, leaves Mera alone on the surface, she rejoins him, again he walks out on Atlantis, but this time Mera stays as Queen. It is stated that he is unacceptable to Atlanteans because he's a half breed, while its ok for them to have a hated xebellian enemy as a ruler. Come on its all mixed up.
AM comics has serious flaws, sticking to it would have hampered the movie just as it can to any future sequel.
As you say, Momoa worked mainly because he was playing himself, any other actor would have been portrayed as the traditional AM, with the bad UNDESERVED reputation of AM, it would have turned to be a failure.
I have been a fan of AM all my life and honestly at times it has been hard to follow the comic version, especially when it had the potential of really being great, it was always dissipated. Just take The current comic issue, It's messed up pretty bad.
Hope any future AM movies don't follow the comic version or it will throw away the success it achieved.