There are a large number of companies that you can buy travel insurance from that will cover medical expenses during a cruise. Be aware that all insurance companies and policies are not created equal. Do your research well because policies in the same price ballpark are NOT always equal when you read fine print.
I found a terrific web site that lets you do a side by side comparison of the companies/policies that fit your criteria. It really made the selection process much much easier than trying to gather all the bits and pieces from a lot of different sources. I'm not sure if a link to this particular site is permissible in this forum so I won't post it openly right now. (BTW, I have no financial interest in the site except for being a satisfied customer.)