Well DC characters are just modern surrogates for greek gods [Superman (Zeus), Aquaman (Poseidon), Batman (Hades), Wonder Woman (Athena), Flash (Hermes), Green Lantern (Apollo)] and have always been written that way so naturally they'd be in a higher tier than your average Marvel in terms of powerset.
RE: Namor though.
Namor also has Hulk level strength in the ocean. He gets weaker outside of water but even then still stronger than most. So in terms of raw physical strength he is indeed superior to Aquaman. The problem with him is that he's also hot headed and arrogant while Aquaman is a more reserved tactician so he could easily capitalize on Namor's rage many times over with any of the other abilities he has that Namor lacks (Ie: his telepathy and hydrokinesis).