The ropes would be a problem if it were towed as a barge. But then Nelson's ropes were all made of hemp - a material that was used in prehistoric times. It wouldn't matter too much if a shallow draught dugout overturned as long as the bluestone was securely pegged in place. The buoyancy of such a vessel would depend on the natural fibre of the trunk rather than being hollow. Dugouts were the most common type of boat in ancient times and we disregard them as an oversight.