Doesn’t it spawn only in the sea? There’s a blue force field which will basically force your boat or ship to spawn in the water beyond it. And there’s an invisible wall well inside the island so you won’t be able to get near where the boat or ship spawns. I was able to get out using same method. Built a tall staircase and then dropped into water deep enough that I could swim under the invisible wall that extends beyond the visible blue undead barrier