My husband is in the insurance business, and we actually read our travel insurance contracts, so I can tell you that they cover a lot and don't cover a lot. My best advice is to ask to read the contract prior to purchasing and then make a decision based on your own individual situation.
Please remember that insurance is marketed and that it may appear that you are covered for many incidents (trip interruption if you are unable to visit the ports that you booked) that are not covered due to some fine print. I had some friends recently who sailed and the itinerary was completely changed. They had some expenses associated with the changes and were unable to receive a refund because other ports were substituted. So they had to pay for some private tours that they had booked that they were unable to go on. There's always a risk that things will not go the way you planned on any trip, but it's regretable that some passengers think that they are covered for incidents of that nature when they aren't. In other words, they may not get the services that they thought they paid for.
In all of the Frommers travel books (probably Frommers Scandinavia or whatever for you), there is a list of all the travel insurance companies, and you can book this separately from your cruise. In other words, you don't have to buy what your travel agent is selling. But be careful because sometimes pre-existing conditions are not covered if you don't purchase at the time you purchase your cruise. If you don't have any pre-existing conditions, that may not be a concern, so just FYI.
Hope that helps,