Young Johnny Storm and Namor are fighting because, well, because of testosterone, really...
But this is early days at Marvel. So instead of just busting up the 'berg, Namor resorts to the age-old comics stratagem of "pulling a brand new power out of his ass":
See, in Marvel's early Silver Age days, it was stated that Namor did indeed "have the powers of all of the creatures that live beneath the sea!" So he would generate electricity like an eel, use the "radar sense" of blind cave fish (yes, I know...) or whatever Stan and/or Jack needed for that particular scene.
Those "powers" were conveniently forgotten when Namor finally got his own series, and never cropped up again, until John Byrne revived the idea in his Namor series in the 90s.
But this is 1963, and Namor still has those powers, and...a puffer fish?!? Really? How the hell is that going to help you when you're about to run into an iceberg?!?!?!
But then again, it's yet one more way in which Namor is infinitely cooler than Aquaman. He can fly; he can stay out of water for longer than an hour (is that still an Aquaman thing?); he's a mutant; he can personally use the powers of sea creatures; and when he wants to summon beasties, he doesn't just make a furrowed brow and think hard--he blows a big-ass sea horn!! Extra bonus--no Garth!!
And if you ever need a hero who can blow up like a puffer fish, Namor is your guy!!
From Strange Tales #107 (1963) as reprinted in Human Torch #7 (1975)