I always buy a policy and ensure it also includes medical evacuation coverage. The cost of an air ambulance is quite substantial. For instance, every time you see your local medevac helicopter flying by it is a minimum of $10,000. I know from personal experience. My wife rode in one after being t-boned in 2006. It probably saved her life. So if a 10 mile ride from an accident to a hospital costs that much, think about flying in a medical jet from Costa Rica to Miami.
People will drop $100 in the cruise ship slot machines or on a couple bottles of wine but not buy insurance. I just don't understand it.
The cruise line policies are usually the worst - both in price and what they cover. Insuremytrip.com is a great website to compare and buy.