Note: I just got the Blue Narciss, so I'm at the beginning of Disc 3 - please avoid spoilers after that point (I don't think this will be a problem)
So, I know that Alexandria and Lindblum had existing (as in, before the game begins) technology to make air-ships that ran on Mist. It was implied that this was the de-facto method of transportation across the Mist continent.
When Zidane and co. are talking with Regent Cid about going to the Outer Continent, he makes it sound like some people know there's "more land out there", but its not common knowledge. When Cid suggested that the party travel to the Outer Continent, I remember Zidane saying something like "well how are we going to cross the ocean?" rather incredulously. Cid also mentions that these continents are largely unexplored.
However, in Disc 3, when the party decides to go hunt Kuja, they pop into a sea-sailing vessel that's implied to be nothing special, and even old technology.
Furthemore, Alexandria is shown to have a full navy when Brahne assaults Kuja at the lifa tree.
So, my question is: If ocean-sailing vessels existed before Mist-airships (as would be intuitive), then what prevented the outer continents from being explored and/or settled, and why are they mysterious to Mist Continent natives?