Interesting.. I wonder if it's the same problem I ran into. On all my Britain to Skara trade runs in the past I always did the same path (100's of times) where I'd eventually end up near the Skara docks by the mainland ferry, move up a little bit and use a boat to cross the small bit of ocean to the island. The one time I tried today my character stopped dead within view of the island shore and my boat kept on sailing. I couldn't move, couldn't recall (because of the trade quest) and the boat wouldn't respond to my commands.
Luckily it hit the shore and I had the plank open so I could hop on the boat that way and then get off. I wonder if there's something odd about Skara and the area surrounding it?