Normally the cruise line will cover all of your health care costs while you are on board with the possible exception of a needed medical evacuation.
Can you be a little more clear about what kind of coverage you are looking for?
If all you need is medical evacuation then American Express has a policy they offer cardholders with is only about $200/year.
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.