@Spish:
Are you sure it's ocean related, rather than a coincidence? Neighboring civs seem to have to reach some size (300+?) before they have enough people to wander around the world. I've seen a total of 1 visitor over half a dozen pocket worlds, but all those worlds have goblins flourishing and everyone else just hanging on. Removing a tavern object (i.e. the named thing, not the zone(s) associated with it) screws up any visitors on the map who came to visit it, and might possibly (pure speculation) screw up future visits as well.
Now, if you've embarked several times in the same world from the same initial pristine world generation save and can see a pattern where the ocean embarks consistently fail to get visitors while neighboring non ocean ones do, that would be fairly convincing, especially if the visited neighbors were further away from other civs than the ocean bordering ones. Note that sequential embarks won't do, since it seems civs have a tendency to collapse (due to rampant warfare exploding on embark [well, probably the transition from world generation to active world] as well as unknown factors leaving large number of sites inhabited but unaligned to civs), so civs healthy on your first embark might no longer be healthy on the next one(s).