if you are injured on a cruise, don't expect the doctor on board to do much to help. medical facilities on board are minimal, and the doctor is an independent contractor to keep the cruise line from getting sued if he does something wrong.
for serious injuries, cruise ships most often drop passengers off at the nearest port for treatment, and sail on without them. it's up to the passenger to find their own way home, plus medical care in non-u.s. ports is not covered by most insurance, plus you don't get any of your money back from the cruise line.
for all of these reasons, talking to your agent about travel insurance before a trip is probably a good idea.