Yes it is worth it. Policies can cover lots of things. You can pretty much customize a policy to suit your needs. Medical considerations are a big factor for us. If you are injured or have some other medical problem while away on a cruise, it can cost a ton of money to evacuate you back to your home. If a medical evacuation helicopter cost me $10,000 for a 15 mile trip last year, what do you think it would cost to fly you home from some island in the Caribbean?
Policies also cover trip cancellation, trip interruption, lost luggage, etc. Lots of things.
Cost depends on how much money you paid for your cruise - including air, hotels, rental cars, etc. It also can depend on your age. Many policies have caveats against pre-existing conditions, but many also waive those rules if you buy their policy within X number of days of making the booking.
You can buy a policy pretty much anytime after making a booking, but as I said above, to get any pre-existing health problems covered you need to buy a policy within a certain time frame. It is common to see a 7-day grace period although some allow 14 days between booking and buying a policy.
See for yourself! Go to Insure My Trip
and you can compare policies and get lots of information.