Well David.
Usually, and I assume it was done as it is almost standard nowadays, these cargoes are carried "on Deck" at shippers risk. That means that anything what happens to the cargo , the carrier (the ship) is not responsible, unless it is gross neglect from their side. Cargoes are further lashed/secured under the observation of the ships command and by surveyors from shore hired by the shipper and/or receiver, special when it comes to expensive cargoes like such a yacht. Once finished loading and securing/lashing the surveyors will issue a report where they confirm that it was done properly and according to standards internationally accepted and recommended. Vessels usually have a lashing book manual, where it is outlined how to lash different things, but usually it is kept to standard cargoes what the vessel is supposed to carry. In this case, that the yacht crew did the job most probably special agreements were met to do so, but I am also in that case pretty sure that cargo surveyors from the insurance/shipper side were also present. Who in the end is responsible depends why it fall over board. That can be a dicey process,but usually it is figured out between insurances !!!!