Namor is a half atlantean, half human. He has the mutant power to fly and because of his human side he can breathe air, and because of his atlantean side he can breathe water. He is relatively stronger than humans and can swim way better than any human can. He carries a large sea trident as well.
Aquaman is a human. His mother died when he was a baby. His father devloped a home under the sea and taught young Aquaman how to breathe underwater. He later taught himself how to speak to fish and other marine life. After that due to nuclear radiation he developed a disadvantage: he had to be in water at least once an hour.
They are both equally skilled at swimming. Aquaman is slightly stronger. Namor can fly and has a trident.
Aquaman has to be in water at least once an hour, Namor doesn't. If the battle between the two were on land, Namor would win. If the battle were in water Aquaman would win.
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