As I type this, 6000 people are locked down on board Costa Smeralda in Italy, due to their being sick passengers on board who had visited Wuhan. They only boarded on the 25th.
Thinking it through, they really need to quarantine the whole ship for 2 weeks. However if any passenger subsequently does show symptoms, that 2-week incubation starts again - another 2 weeks - and so on, so they'd better hope that confining those poor passengers to quarters works... Also the ship had already visited a number of ports during the cruise...
I have fairly rapidly come to the conclusion that cruising isn't really possible at the current time, and I'm not the only one - Carnival shares are down 7% already.