Bear with me. Here's the idea:
You get a message/offer from an NPC that a valuable "secret shipment" is located inside a barrel on a ship currently sailing (on your SoT server).
This quest/mission, if accepted, would instantly place unknown contents in a "special barrel" on board an unsuspecting player's ship (could use one of many currently unused barrels on ships).
The "secret barrel" ship can either be visible to the player with the quest (aka; reaper's flag style), or if not, the NPC could give a clue/s as to the ships current coordinates, or a nearby island where target ship can be found at any given time.
One strict condition of fulfilling the quest is that the mission remain clandestine; iow, the ship must not be sunk, otherwise the mission is considered a failure. (Pirates being murdered or not, can be an condition of success as well.)
The player accepting the secret mission must then somehow board the target ship and retrieve the contents in the "secret barrel", again, without the ship getting sunk/ or scuttled.
Accepting the quest from an NPC may cost a large sum of gold, but the reward/s for success should be much greater.
Of course, the special barrel contents should not be known or accessible to the player carrying the "secret barrel".
The only way for the secret barrel mission to become a failure is if the players with the secret barrel somehow discover they have one, and decide to scuttle their vessel, or kill the secret intruder before he succeeds.
If the secret contents are successfully retrieved (without sinking the vessel) and delivered to the original NPC who offered the mission, the reward is collected.
What do you think?