This is a mismatch. Under water or not, this doesn't affect Superman in the slightest. Namor is of course at his best in the ocean, but that's still quite below Superman.
Superman breaks him in half with only one hand.
DP
Demonic PhoenixSenior Member
Superman could just take namor to land and spank him upside down, but because this in the water...he'll just spank him upside down again.