I'll add one note about Cruise based trip insurance.
Be sure you have a rider on your insurance that covers medical evec and emergency treatment.
Most US based health plans do NOT cover medical expenses incurred while in International Waters or on Foreign Soil. And, forbid, anything were to happen and you needed to be transported to a larger medical center through an Air Ambulance, the cost can run 50 - 100k, easy.
DCL's trip insurance includes this rider by default. Other, cheaper, third party options may include it as a policy endorsement, not as part of a default quote.
For parks, there is no need, as long as you don't mind paying $50 and rebooking the rest of the balance forward to future visit.
Airlines, yes, get the insurance unless you are willing to push forward the value with a change fee. It's cheap, but it's cheaper than change fees and certainly refunds.
One other really teeny note...some more recent US based healthcare plans have limited their risk pools to specific states. Meaning, if you are out of state, only Emergency Services are fully covered when you are out of state. These provisions are being slid into policies starting about a year ago, and are not really advertised. So, be sure that you read your plan documents (or call and ask your Insurance Agent / HR Person if it's through work).