I believe ...
At departure the ship just keeps off anybody who tests positive. So they sail with no negatives.
The tests are so poor that falses are inevitable.
Any tests during the sailing will result in spinning the roulette wheel. There will always false positives if you test enough.
Industry under the odds and toss of the dice will lose.
The benchmark MUST change. Tests cannot be absolute.
I for one will not board any ship that can leave me at any port of call and force me to make my costly way home, based on testing.
I can't imagine anybody going on a cruise in these conditions.