In the lock

Two boys sit on this boat that is headed downstream as indicated by the fact that the back of the boat is at the upstream end of the lock. The water has been released from the lock, lowering its level in the lock. When that process is complete and the water in the lock is at the same level as the water in the downstream level of the canal, the large mitre gates at the lower end of the lock will be opened to let the boat out.