Has any explanation ever been given for Namor being called the Sub-Mariner?

Is that an Atlantean title? Unlikely, since it sounds English.
I have a vague memory of him claiming it was a stupid title the surface-dwellers gave him. I may be confusing him with Aquaman though.

On a side note...I just dug out Marvel Treasury Edition Giant Superhero Team-Up for my 5 yr old son. It's always nice to be reminded how great John Buscema was. He did The Thing vs Namor & Spider-Man vs The Silver Surfer.

It was also nice to be reminded that Roy Thomas could be a good(great) writer.

Kirby was his usual awful self doing The Hulk vs Thor. I just don't get why so many people love him.