I said it helps, not that it solves the problem.It is theater by means of making them "feel" safer. How many are testing negative 1-2 days prior but don't have the viral load to test positive yet? They breeze through onto the boat and then expose the rest of the ship. Covid is a non issue unless you have a form of immune deficiency or have had your immune system shattered due to cancer regardless of vaccination status. This isn't 2.5 years ago when it was causing 2 weeks of walking death. Now it is a mild cold for all classes.
Back on topic... buy trip insurance and keep all your receipts for the trip. If you end up with coldvid then jump through the hoops and make a claim.
If the tests are self administered take one at home and keep it in a pocket. Show them the stick if asked.