It happened in an issue of Invaders, which was set in the 40s. Hitler's top scientists pulled Thor through some dimensional aperture, bringing him from Asgard to Europe and sending him after the Invaders. Thor realized he'd been duped, said goodbye and told everyone that his dad Odin would erase their memories, which he did.
Namor would get punked by Thor...Underwater Namor MAY have a chance. This is of course assuming that a body of water is nearby and that Namor is somehow able to get Thor underwater.
This is the same as saying Superman could beat anyone if he was close to the Sun?
Being underwater doesn't make Namor invincible - Thor will take this.
__________________
-
"Clark is the most human of us all. Then...he shoots fire from the skies, and it is difficult not to think of him as a God."