I've got a similar problem, just in my case Tarquin never even left the ship. He just stands next to the rowing boat on the Vengeance, with no intention of visiting the graveyard or giving me any quests.
I entered the graveyard through the cellar in the mansion, so maybe that messed up his spawn. Additionally I found the first piece of the sword before getting the quest. Talking to him with both items in my possession doesn't trigger anything either.
I really hope there'll be another hotfix soon, clearing up these problems, as it keeps me from progressing in this wonderful game.
