The deep water towards the middle and closer towards the outer range boundary are the most dangerous with the potential for strange weather, be it storms or the like, popping up. This is also where you can encounter or catch sight of classic sea monsters. There are more peaceful times out there as well, but the likeliness of disaster occurring is high. You can take most typical fantasy oceans as inspiration for this.

It's possible to go past the outer range into the wastelands, but then you got the wasteland to contend with it and it gets worse and more dangerous. It's likely you can reach about a mile out before giant megalodon sharks jump out at the ship.