You're right, they can't operate expecting to have no cases, I don't see that happening anytime soon. IF (appropriately caveated) they can get rapid testing figured out and have sufficient tests per voyage, that would go a long way towards the mitigation and control while on-board. You may have to test every day, certainly returning from a port of call. Increase the medical center, increase staff to ensure passengers quarantine (boy that would be a fun job, staring at a cabin door all day).
My worry is less about the ship than it is with how the ports react to a ship with cases. It's one thing for a port of call to deny a ship, but a home port is another matter. Hopefully I'm not naive in the assumption that the cruise lines are discussing COVID protocols with the home ports.