Would wording like this be sufficient?……are eliminated by being hit with all cannons a ship has available.
I think so. Yes, Castaway is already an extremely good UT and arguably OP.
Point Value: 1
Ability: This equipment takes up no cargo space. When this ship explores a wild island, you may eliminate this equipment to eliminate any Unique Treasures on the island before their effects are applied.
Flavor text: This small black box is an innovative tool filled with powerful explosives used to clear an island of potential hazards to exploration.
Historical Custom of the Day #297
Ability: Ship treasure trading. This ship’s crew cannot be captured. This crew can use his abilities on land.
Link: Stede Bonnet, David Herriot
You can find my custom sets here: Pirates of the Age of Sail and Pirates of the Epic Seas