Our last cruise we did tell them it was for insulin but they did not waive the cost. Still very inexpensive. True they are only "coolers", but we take a thermometer for the cooler, and we keep the ice bucket full of ice uncovered in the cooler which keeps the temperature about 39-47 degree F. and the room stewards fill the ice twice a day.
Keeping the insulin at the medical center did not work for us -- as their hours are so short and inconsistent! We found a way to make it work!