Ok, go float at the beach and see how much of your head is out of the water. We are not talking about the Dead Sea here. I don't think I'd describe average ocean salinity as providing much better buoyancy. I lived by the ocean all my life and spent plenty of time in it. I am quite sure if I was ever alone in the water and went unconscious that the outcome would be bleak.
This issue really doesn't require some complicated scientific experiment to figure out what is most likely to happen going overboard from a cruise ship.