I've been curious about this for quite some time. If you go to any coastal cities in Arena, you can find a large body of water which is presumably the ocean. You can kind of see the skybox (or some weird flattened "distant land") beyond it, but an invisible barrier seems to stop you from progressing very far.
If you use Levitation (as in Arena it's mostly just for surmounting high walls and floating over water), you can get closer than you can by swimming - only to have the game crash once you get particularly close to it.
I've been curious about this for a while. Anybody ever notice this?