I really think that Spidey is going to have a lot of trouble with Namor. A whole lot more than with some of the chumps he's put up against in these threads but if he's not holding back he's far more powerful than he's shown to be. The problem is that you would have to imagine what he'd be like in a killing rage and how that lack of concern would affect what he would do to an enemy in a fight. In the comics 99% of the time he's holding back so you really do have to imagine. Firelord got beat down for holding back until he didn't even know what was going on. Spidey was in a panic when he jumped at firelord and let it all hang out
in desperation and rage. The end result was unconsciousness before he could get mad enough to stop holding back trying to secure a victory in direct combat. Namor does not dominate this fight by any means and there are few characters in Marvel or DC that will just whip Spideys as*
like it aint no thing. He can stay a step ahead of almost any attack and is usually defeated by himself. It has to be harder to fight someone who's trying to kill you when you're DETERMINED not to kill them. But he still beats some terribly powerful characters so he's not to be taken lightly in a one on one with any character that can tire or be damaged by the environment. In a desert Spidey will very likely lose because he will tire faster than namor. In the city I think that it could realistically go either way just because Spidey will be chased and that's not good for Subby at all. Also spider-man can take a lot of punishment and rarely takes the full force of a killer punch. 50\50