I have blogged about insurance before. I feel it is important when you are traveling out of the country. The expense just to get you off the ship and home if there is a medical emergency can be incredibly high. This really is a personal decision though. Some cruise lines are now offering a "cancel for any reason" provision either with the insurance they are offering or in conjunction with the insurance they offer. This means if you need to cancel the cruise for a non-covered reason and you are within the penalty period, there will be some concessions. On Disney Cruise Lines, once the insurance company has denied your claim, you will have the ability to apply the penalty portion to a future sail date (not including the insurance cost). On Celebrity this amount is 75% of the penalty portion.
Many of the policies offered through the cruise lines do not allow for coverage in the case of a pre-existing condition. The cruise lines that offer a "cancel for any reason" provision will still have you covered at least partially, but otherwise, you will want to know when the coverage takes effect (in many cases it's when the final payment is made) and what will be covered. Otherwise, you may want to look at independent travel insurance.
A good place to start for this insurance is www.insuremytrip.com. They will ask you a few questions about the trip and then give you quotes from a number of different companies. This will give you a chance to compare policies and find the one that fits your needs. Using an independent company can also be a good idea when you are combining a number of activities together. Let's say you are doing back to back cruises with a pre and post stay. Booking through the cruise line you will need two policies (one for each cruise) and this likely will not cover your pre or post stay. With an independent contract, you can combine your entire trip together in one policy.
If you do want to be covered for any pre-existing conditions and are purchasing an independent policy, you will want to look into this very soon after the deposit is made on the cruise. Some will waive the pre-existing clause if you add within a certain number of days after the deposit is paid. The problem with an independent policy or any policy that is non-refundable once it is added to the cruise package is that if you change your date, you may very well have to pay for insurance coverage again.