So, I've always been a fan of villain fights, so I'm curious who would win, the nemesis of Namor, or that of Aquaman.

Attuma the Barbarian:

http://www.leaderslair.com/marvelvillains/attuma.html

Black Manta:

http://www.rapsheet.co.uk/RapSheetMain/Character.asp?UniqueId=613

The fight takes place in neutral waters, and neither has their private army to back them up.