I think having a boat vault/locked cage with a limit on items that's allowed to be placed in on your boat could be a good asset.
Having the 'captain' of the captained boat drop a vault key similar to the Sea Fort when killed, giving access to the Thieves of the Sea to the captains most valuable treasure.
Could have the same mechanic / code as the loot to the merm statue.
Have the same key drop mechanic as the sea fort captain, acquire the space below the ships stairs for the vault/cage
