I've only ever had one instance on a cruise where I had to make use of the ship's doctor and that was on Royal Caribbean's Soveriegn of the Seas.
My son took ill with what we thought was bronchitis. It cost $250 just to see the ship's doctor who did nothing but give him a bottle of cough syrup for an additional $25!!
Upon debarkation, our family doctor diagnosed our son with pneumonia and prescribed antibiotics.
I don't know if all RCCL ships operate that way, but the medical care on the Sovereign left a rather unpleasant memory.