I'd suggest some more detail about the layout, purposes, and defenses of each area. Are there interesting NPCs to encounter in each area?
More could be done about how they control the island's movement: Are there areas dedicated to that? Are there cultists responsible for maintaining the magic?
How can someone land on the island and what defenses will they encounter?
What sort of doom awaits in the Forest of Death? Do the cultists hold ceremonies at the Tomb of Fallen Jarvin? Is it defended by undead or other horrors? Perhaps Jarvin stole precious relics of the church when he fell from grace: Useless to the cultists, they are believed to have been entombed with him. Go to Comment
I'm afraid no matter what kind of ghost ship is written nowadays, it will always be compared with the one (well two) of the Pirates. Luckily, this is more thant a simple mix of the two, and it has a few extra thoughts. Go to Comment
Unfortunately Disney's Pirate's movies were popular. Otherwise this would be fairly original. Regardless, it still sounds like a lot of fun to play. And sea Quests are rare.
5-Plot Twist Idea: The treasure is there, but what if when the relic is removed from the box/ships hold/ship, Deathwail starts to disintegrate. (keeps the PCs from gorging on treasure)the disintegration could be described as pulling the souls from the walls of the ship into the item. opening up holes in the hull, where they once were. Go to Comment
Many games draw moral lines in bold colors, where the real world is not so easy to categorize. Suppose that the player characters are faced with an overwhelming foe? Even unsavory allies such as orcish barbarians may be better than no allies at all. More disturbing, these allies may be honestly friendly to the PCs when all is done, overcoming barriers of race and religion. Will the PCs remain friendly with the bloodthirsty humanoid tribesmen when their mutual foes are defeated? Some would expect the tribes to betray them, but after the characters have honestly won their respect, even orcs may not be all bad.