I often recommend that people compare policies at insuremytrip.com, where you can line them up and see what the best policy is for you.
I look for policies that have trip interruption, trip cancellation, medical payments, and medical evacuation. It may be too late for you to get pre-existing condition waivers, which is useful if one of the guests has an ongoing medical problem that could flare up just as the cruise begins. This waiver is usually available only if you buy insurance within 10 to 15 days of the cruise being booked. It can vary by company. However, you can buy policies which will pay you back if the cruise has to be cancelled for any reason. And yes, they cost more.
Don't dismiss the medical evacuation coverage need. If someone fell ill in a foreign port and needed to be flown home in a medical evacuation jet it can cost $50,000 to $100,000 or more. For perspective, I had to pay $10,000 for a medical helicopter for my wife a few years ago, and it was a 15 mile flight. It is not uncommon to learn a person's medical insurance will not cover evacuations, or will only cover a percentage of it.
I have found policies from Travel Insured, Travelex, and CSA to offer good coverage for reasonable prices.