(Just following your apparently Wonder-inspired threads...)
It would be a good fight. Ultimately, I'd say Donna would get the majority, but Namor definitely gets a good number of wins. Donna gets the speed/maneuverability advantage, while Namor gets the "I'm *NOT* losing to a GIRL...* advantage (while that sounds funny, I mean it quite seriously -- I just can't see him letting himself get beat by the bargain WW...).