The so-called "turncoat" Ronald Marchaterre, believes Chiel Hesslewood has sold out to the British. He cannot go to the town boss with this information, however, because the boss will not take Marchaterre's word for it. The "turncoat" needs you to get onboard a nearby ship and confirm his suspicions, since Boss Daudegaud will not believe him without a second, more trustworthy witness.
This is a multi-part mission.
The first part is a swashbuckling mission aboard a ship map with level 29 opponents.
There are nine allied (for now) British NPCs on the deck, as well as a mission NPC named William Watterson. Talking to him will trigger a conversation where you are given three options, all of which are wrong and will turn the nine NPCs hostile. William Watterson will disappear.
None of the NPCs are lieutenant level, and if you are adequately leveled, you can group everyone up and take them down with AOE.
Defeating them will trigger the next phase, where you are directed to enter the cabin at the aft of the ship. William Watterson is in the port room, behind you. Talking to him will finish the instance, and you can return to port. Talk to the town boss to progress the mission.
The second part is a difficult ship combat mission on a coast map with level 25-35 British frigates. Your single ally will be in a pathetic starter ship, and will sink quite early on in the engagement, as you will be fighting Raas and Capriceuxes. You begin by fighting two, and halfway through that fight, about 4-5 minutes in, two more will spawn. This is a difficult fight, and you may need to kite away from the opponents to heal up.
Once you sink the ship, return to your NPC ally (who didn't die from sinking) in town.
Defeat All Enemy Crew (9/9)
Talk to the Captain Inside the Cabin
Defeat All Enemies (2/2)
Defeat All Enemies (2/4)