I don't know if I have an answer for you, but here's some info based on my past few days of researching this stuff.
If you need air to get to the ship, and do not buy air through the cruise line, be sure to get 3rd party insurance to cover flight and cruise. There are several companies that have been mentioned on these boards, CSA, TravelGuard, Access America, Travelex, etc.
For my purposes, prices are generally under $200 for 2 people, usually about $170 or so. Access America has 2 versions of their brochures, one would cost as low as $103. However, my main concern is my flight getting me to the ship on time. I have found that some companies (A.A.) only cover delay if it is weather related, which wasn't enough of a reason for me so I think I will spend more for a different plan. Travelex appears to be more generous in covering delays for various reasons. The coverage for delay varies usually $500pp, but CSA will cover $750pp.
Depending on what cruise line and what you know about their financial stability and how far off your cruise is, bankruptcy may or may not be an issue. CSA doesn't cover bankruptcy, others do.
Baggage delay/loss is about the same, but with any amount of coverage, I would still be livid about missing luggage so that's sort of a non issue for me to consider.
I think all the companies I mentioned will cover pre-existing medical conditions as long as you comply with certain conditions - for the most part, applying for coverage right after making deposit for the trip. Some companies have policies to cover pre-existing just for the travelers OR travelers and their family members.
Medical expense coverages range from $5000 (Travelex) up to $20,000 (A.A.).
I suggest choosing the company that best covers whatever you are most worried about. Good luck.
I agree with the other messages, but would add one comment. The insurance offered by some of the cruise lines (Celebrity & Princes that I know for sure) includes a 75% credit if you cancel at anytime for what otherwise would be an "uninsured" reason. This may or may not be good enough reason for you to pay the extra costs of cruise line provided insurance, but it is a good reason to considered them. It all depends on your particular situation. For me, this benefit is very important.
I went with Traveguard. The main reason being that if you purchase within 7 days of putting down your deposit they waive the pre-existing condition clause. And this is something I needed to be covered for.