In their most famous fight, Spider-Man was clearly having the upper hand, while Namor was getting weaker by the second (but it ended of course a draw like so many battles). And this was on a ship in the middle of the sea !
Imagine what Spider-Man would do if Namor was so stupid to attack him somewhere in New York, the natural habitat of Spider-Man...
In water, Namor is a powerhouse, he's Class 100 in water.
Out of water he's Class 70, superhuman reflexes and reaction time, resistance to injury, flight, and superhuman stamina.
Spidey anyway has show his own against him or strongest foes, but i think that the fight would be like this:
Namor exit from the sea flying when a web is stucked on his face.He remove it and says: Who's the idiot that has insult the Son Of The Vengeance?!!
Uh, excuse me Sub-Mariner, i was swinging with my web but
Wall-Crawler it's the last time you made a trick at me!
Namor in this fight wouldn't fight like a stupid guy anyway, yes enraged but not stupid, he just want to wipe the floor with Spidey, that has to fight with no-mercy cause Namor wouldn't show mercy to him.
Or probably another situation would be Namor has to kill Spidey cause Namorita would be killed by Morlun if he don't kill the wall-crawler.
Anyway a give this to Namor, but only after Spidey give him a run for his money using at top his reflexes and spider-sense, but Namor would finally knock-down him.
for the files spiderman can lift 20 tons now and better physical stats overall.
but still it woult be a really hard fight for peter. if he can fight namor long enough on land so that namor gets weaker he could win.
it just a question of durability and his speed.
but in the most cases i give namor the mayority.
8/10 Namor