I wouldn't mind the Rock, but I do see a minor problem with his casting, and it's completely cosmetic/budgetary. The Rock now has these elaborate Hawaiian-tribal tattoos all over him, right? I may be wrong but I think he does. So think about it. Namor never wears a shirt, so every time that he was on screen they would have to digitally remove the tattoos. That means CGI work on every single shot of Namor.
In my opinion that would definately bump up the cost of the effects in what should already be an effects-laden film. I may be wrong. It may not make that much of a difference. But if I were producing a big-budget, underwater special effects movie, that extra bit of money devoted to something completely cosmetic would play a part in my decision.