Was on boat with my necro, when I cleared boat and was on my way down from the upper deck I saw that the Captain and his crew had popped.
Started to pull and had some adds, but only 2 adds. 2 others had gone down on the dock while I was fighting so I continued to pull the ones left, while fighting 1 more left the boat. When the ones on the boat, except Captain, was dead I thought I would wait for the others to return thinking the Captain wouldn't be targetable untill all were dead. But when I got close to him he jumped me, not much of a fight though, the 3 that had left the boat came back just after I killed the Captain so killed them too. So It seems you don't have to kill all the others before the Captain gets targetable. Not recomending anyone to camp him though, got shawl and man what crappy loot for this encounter, got the Talisman too that is part of a quite time consuming quest that gives a charm that is even worse than the shawl. Someone at SOE should be ashamed of this one.