Just remember, you asked.
Much about this contest has to do with the terrain. Should they both be fighting on land (above water), the clear winner would probably be Namor if the battle took place for over an hour. Namor has spent decades outside of the water and Aquaman would die after an hour's time.
Underwater is a different story.
Both combatants are almost evenly matched. The strength of both characters has to be superhuman. Namor's has been documented at Class 100 tons and while Aquaman's has not, we must remember that he can swim and survive (thrive even) at the bottom of the deepest seas where an ill-designed bathyscaphe would get crushed like an eggshell from the tremendous pressure.
So both characters are pretty strong. Where Aquaman edges out is in his talent, use, and assistance of his underwater connection to "the clear" aka his ability to communicate and command sea life. He's even been able to use it against human beings by shutting down the one area of our brain we kept from crawling out of the sea (evolution-wise). Should he pull that trick with Namor, the fight is over.
Namor does have combat training with his years of fighting with and against Captain America and the Invaders. So, if it came to actual fisticuffs his decades of time spent with fighting teams (and his own armies) as well as his alliance with the Defenders. Aquaman has never really been in knockdown dragout fight with many people - and the ones we know of have taken him. Let's remember that Aquaman lost his hand to piranha during a fight with Charybdis - who was no Namor.
Let me put it like this:
Land battle - Namor wins
Underwater fight - Namor wins
Underwater to the death - Aquaman