Although we tell everyone that you can do literally "everything," working at sea still runs by old rules. There are standards for Maritime Pre-Employment Medical Examinations (PEME) that are almost universally required. Ship owners and insurers (mostly insurers) don't want to take any chances. If something goes wrong, most cruise ships, while they may even have a doctor on board, they are not prepared to deal with DKA or a severe hypo. And they often just cannot call in quick help. So insulin dependent diabetics are mostly unable to pass the PEME. And for some companies, they may still not approve you if you are just on oral medications.
I know it seems whacked in this era of the American with Disabilities Act, but cruise ships are usually "flagged" in other nations which are "lenient," even if the cruise headquarters is in the US. So often the rules aren't even US rules. And once the ship is out to sea, it is international rules.
ps. I had my insulin all frozen on a larger cruise ship (in the "infirmary fridge") and while they had a Physicians Assistance on board and were able to provide vials of NPH and R and a couple of syringes, that was "it." If I had been in DKA, I might have been in real trouble.