The disease has torn through the people on board the Dutch ship, with initially four people reported sick during Friday night, rising to 30 and then 60 within just a few hours on Saturday.
Those affected have severe vomiting and diarrhoea, and on Saturday many were being given liquids intravenously to prevent dehydration. Around 60 officials are at the scene including three emergency doctors and representatives of the regional health authority.
Some of the passengers had visited a Christmas market on Friday evening, but no connection between the illness on the ship and that trip has yet been made.
There are 145 passengers and 42 crew members on the ship, which has been partially reorganised into a hospital ship, rather than a pleasure boat.