I think this was the second proper Marvel Annual, by which I mean an oversized, standalone story that “mattered” to the overall continuity of the main book, containing original material. Interestingly enough, the annual also reprinted the team’s origin from FF#1, but slightly altered it to match their visual appearance at the time the Annual was published.
In the story, Reed Richards takes the family on a vacation cruise through an area of the ocean known for sea monsters. Of course, Sub Mariner is behind it all. Fun ensues. We also get a bit of Namor’s origin, showing how he’s half-human and half-Atlantean. It’s a pretty significant story, since it lays the foundation for future wars between Atlantis and the surface world.
Namor had been around during The Golden Age, and was the star of Marvel Comics #1. There’s a bit of a reference to that, but overall the Marvel Universe seems to begin with the Silver Age.
Creators: Stan and Jack
First appearances: Atlantis and Atlanteans like Lady Dorma and Krang.
For the complete history of the MU, year by year, go here.