I think you are seeing this from the wrong angle: do countries in the Carib., which may have limited ICU facilities, really want daily influxes of thousands of Americans when infection rates are still high in the U.S. They are desperate for the tourism income, but must be worried that interaction with cruise guests will infect their population.
I think this is why there has been discussion of 3 & 4 day cruises. Passengers are checked on boarding, then unless they get sick the first 24 hours they are only 2-3 days away from port. And can quickly get back to the US.