It's great if someone has a health insurance plan that covers them overseas.
The problem is that the vast majority of health insurance policies do not and people who are not covered believe they are.
I believe that the previous posters have verified their coverage with their insurance providers but if you "think" you are covered outside the country you need to verify it. Call your agent/provider and specifically ask them if you are. Believing or thinking you're covered is worthless. You have to know you are covered.
If you are covered that's great; you save a few hundred dollars on each cruise. If you aren't covered and don't take out insurance then you can lose a couple hundred thousand dollars.