Yeah, that's always be a bit of a mystery to me too. The Atlas Era Masterwork credits Carl Burgos as cover artist with an asterisk next to his name, meaning that they aren't positive but that's their best guess. Odds are that he wasn't COMPLETELY up to speed on Namora's particular skill set or specific features. It IS her, based on some costume changes visible during the Atlas era, but she appears to not have her ankle wings and has lost her belt with the big "N" buckle that we find on the interior pages. No panel depicted on the cover appears in this or any surrounding Sub-Mariner issue. If I had to guess why the ropes are holding her, I'd guess that maybe she's been out of the water for too long and has been substantially weakened by that. Just a guess.