Sub Mariner is best when he’s teamed up with someone else, so in this story Marvel tries to get some of that Namor fame to rub off on their newest character, Captain Marvel. But before we get to the mix of cosmic and sea, let’s admire that cover.
Namor and Attuma fight, with the freedom of the Atlantean people in the balance. Nothing to do with the Mar-Vell tie in, which happens after.
Captain Mar-Vell is testing a rocket, which crashes into the ocean. So they fight.
The Sub-Mariner issue is among the top 10 John Buscema comics of all time.