I expect that clause means 'if you test positive we will lock you in a cabin of our choosing, not the nice comfortable one you booked, until we are able to offload you at the next port of call, where you won't have any cover for the costs if you need medical treatment unless you are able to claim them on travel insurance. Your flight home from wherever we offload you will be in Y, and it will be up to you to try to get a refund of whatever flights you had booked when you thought you were getting off the cruise at its final destination rather than part-way through. We hope that giving you a full refund of your cruise fare means you will not be so furious about this that you never cruise with us again'.