Being a physician, I can tell you that I am 100% confident that the medical facilities and personnel on the ship are capable of handling most any emergency. It is true that they are not a level 1 trauma center, but I do know that the physicians are well qualified to handle most any situation. The cruise lines only hire Doctors with E.R., family medicine or internal medicine training and won't hire anyone who is not completely qualified by both training and experience.
I can tell you that while diabetes is a very serious illness, it is generally not difficult to manage in the short term. Most of the problems a doctors runs into with diabetes are related to long-term management and should not be a major deal during a one week cruise.
Take care and best of luck,