Coincidentally the only time I've ever seen a sunken cilled boat in person was on this section of canal a couple of years ago, in Wharton's Lock a couple of miles from this incident. It was a Canaltime boat and it was badly sunk having rolled to about 100 degrees angle of bank. The two occupants were standing by the lock as dry as a bone.....turns out they both got off and tried roping the boat in the lock and didn't notice it cilling. They were also both half cut (from the night before in my estimation) despite it being only 11am. Not saying this is what happened here, but it's all to easy to sink a boat in a lock if you're not concentrating.