The Submariner is not in the upper-most class of strength, he's in the notch below that guys like Ben Grim belong too. 

There's a classic Roger Stern/John Buscema/Tom Palmer Avengers issue where both Namor and Hercules are being held in place while at the bottom of the ocean, and Hercules' strength eventually allows him to move, whereas the Submariner, in his ocean element, can't. He's simply not as strong as Thor, Hercules, or The strongest one there is, aka The Hulk.

Sometimes, I got the impression that Ben, especially when the Hulk was around, was only about as strong as Spider-Man, on the same sort of 'proportional' level('The proportionate strength of  several other scrappy street-fighters from Brooklyn who were hit with cosmic rays.')

As a result, just about every Hulk v. Thing bttle I ever read was a disappointment. Ben was a guy who could trash pretty much all of the 'street-level' heroes, and quite a few of the Avengers,as well as some other teams if he was in a bad mood/under someone's influence( I used to wish DC and Marvel had done a crossover where he found himself in the JLA satellite, facing the whole 'Bronze Age' roster of the team).

But, he was always treated as if the Hulk were his 'big brother' beating the crap out of him so quickly, Hulk would lose interest!

And I agree, Namor should be a lot more powerful than he's been depicted, if only he'd been written properly. But then, writers had the same problem with Aquamn, and it is very similar to 'Popeye syndrome'..without the necessary 'ingredient'/plot device, the characters are pushovers.