Alereic

I had this problem, too, and stumbled across the answer. If you can sneak over to the "exit" to Delver's Row, use it and return to the Undercroft. Immediately below that point is a Brawler. Walk past him and he'll ask what you're doing there. With a high enough Bluff you can claim you're there to meet his captain, and after talking to the Captain you're considered neutral and can leave in the boat. I think this is either an oversight or a lucky break, but it will get you back to Queen's Berth.