I don't think you will have a problem getting insulin in any of the usual cruising destinations.
As long as your diabetes is under control, and as long as you don't have uncontrolled hypertension, you should do well with the cruising lifestyle. If you are a brittle diabetic with a propensity for ketoacidosis, then you should have someone with you who knows about diabetes when you are on
passage. If you singlehand and your diabetes gets out of whack
offshore, you could be in trouble. But if there is someone who can
monitor your diabetes in a pinch, you should be fine.
I treat a lot of diabetics in my practice, and the thing that causes the biggest and most common serious problem is infections in the skin that progress to cellulitis. MERSA is particularly a serious problem.
If your diabetes is under control and if you are young, this may be your golden opportunity to go sailing. Later on in life, diabetic complications may limit your ability to
cruise and sail
offshore.