The Story of the Alter Rebbe requesting from the gentile to stop the boat so he could say Kiddush Levana (paragraph 7), stresses the point that the Alter Rebbe would only say Kiddush Levana once the boat was stopped.
What is the reason for specifically stopping the boat?
I have looked over the halachot of Kiddush Levana, and it does not mention any halachot regarding moving, other than having to put your two feet together. I am not looking for the answer that you can't daven on a camel or a horse (which I don't have the source for that right now), since halachically you can daven on an airplane, a train, or a boat.