There have been several stories posted on this board through the years aboutthe ship being diverted to an extra stop to evacuate an extremely ill passenger. I remember reading in the trip reports this week that they did an air evacuation from the Magic during the past month.
The ships medical facilities are quite good, but they are not a big city ER. They will stabilize and then get a patient to an appropriate facility. My sister is in medicine and looked into working on cruise ships - interesting thing she discovered was that Disney originally was slanted towards pediatrics, but now is looking for both pediatrics and elderly due to the number of multigeneration families traveling together. Princess used to look just for internists/elderly specialists, but now has peditrics capable people on some of their Caribean and Alaska ships.
First, verify with your doctor the degree of risk taking the vacation. If they are against it, please try to be cautious and consider postponing.
Second, verify that your trip insurance covers emergency evacuation. When my Dad was in his late 70s and early 80s he continued to travel with my sister -
DCL, Princess to Alaska, planned a trip to Europe. The cost of air evacuations are very high - upwards of $25000 from Europe to the US. We made sure that we always had evacuation coverage to make sure that he could get back to the USA.
Third, Let the medical staff know that you have an issue. My Dad developed some mobility issues the week before we went of a
Disney cruise 4 years ago. They noticed this during the Lifeboat drill and talked with us about an alternative plan in case of an emergency. They were very proactive about making sure he would not be at risk in an emergency and even called to see how he was doing/enjoying the trip mid week. (It was clear to both my sister and I it was really a medical check, but they covered so he didn't realize he was being checked in on).
Have a good trip and best of luck