All the companions are sourcerers, whom you escape with. The prototype being shown now has the party ending up on an island after being shipwrecked escaping from prison. I suppose there could be some distance between Fort Joy and the nearest port, but I don't imagine there would be a lot of sourcerer hangouts between the two, or there would be much selection of other sourcerers at the docks also fleeing the Divine Order's purges.
Isn't the overriding goal to 'escape to lands controlled by the Black Ring'? That's a reason enough for other escaped Sourcerers to be traveling in the same direction.
I can see a way for some companions who didn't make it with you onto the boat or something to be in other areas of the game, especially past the first island. We haven't seen the escape sequence. We don't know how many companions are available to choose from the start (probably not that many, though. D:OS 1 only had 4, and two of those took several more months after release to add).
Maybe there's a second boat and ours just got unlucky and crashed. Who knows?
Freedom and Urgency
As for a sense of urgency. I don't particularly care either way. A sense of urgency is great for creating tension and an atmosphere, but that is also at odds with exploration-based gameplay. D:OS is a game about freeform exploration, and putting urgent timers on everything would kill that part off. The boat with the Divine Magisters will arrive "tomorrow", but tomorrow will never come. It doesn't fit with the desired gameplay experience. Freedom and urgency are opposing forces in terms of gameplay.
You can only put a sense of urgency into D:OS by restricting or removing some of the freedom you get. The Luculla Mines are the key example. You lose the ability to travel or escape in a pretty arbitrary manner basically for the benefit of a two-minute or four-turn escape sequence. It was a little clunky.