Mark has been diabetic for about 20 years, and hasn't ever taken very good care of himself; he's paying the price now, and on our last Mariner cruise he watched what he ate FAR more closely. At every meal there was at least one sugar-free dessert, and most of them were yummy.
He was able to get Canada Dry Diet Ginger Ale for the first time ever, and is crushed that we can't find it here. The dinner menu has a couple of different sections with simpler, low-sodium, etc., dishes.
Plan to go and enjoy; he'll do fine.