reef depot
This is a minor one, but while exploring the first area, there were several times where I saw something that looked interesting, in the distance, but while heading towards it I was told there was nothing in that direction and forced back. The thing that made this experience frustrating was how unexpected and sudden it felt. Having a visual indicator that A there is an edge to the map, and B I'm coming up on it would do a lot to solve that problem.
Ideally, map borders would be defined by diegetic boundaries, like large bodies of water or sheer cliffs, but with a map this large and open, that's not always going to be possible or feasible. In those cases, I'd recommend some form of environmental effect, that can be seen before hitting what is, effectively, an invisible wall. Thick banks of fog are fairly traditional, but there are probably other solutions as well.
deft horizon