Fairly easily, actually. He used a little leverage to get out from under the monster's crushing hand (describing the technique as something similar to judo) then formed a twister around it.
Namor forces that trap door open and what does he find? Exactly what you'd expect: A talking fish with a diamond in its mouth.
Meanwhile, his lady love Dorma is in a pickle as well. Krang has grown tired of not even reaching friend-zone with her so he has her banished to the cave of the Faceless Ones.
As a result, Namor senses a disturbance in the force...
Anyway, Namor has 99 problems but Dorma ain't one. He's a prince and he has to go on with his quest. It is what it is.
When he finds the diamonds though, it doesn't really strike him right away that there just might be a good reason why they're called "diamonds of doom".
As you can see from the cover, Namor shared this book with the Hulk so the story is only 12 pages long. We're left with a cliffhanger but if Namor was able to defeat the dreaded Seaweed Man, surely he'll find a clever way of overcoming the devastating effect of the diamonds of doom!!