I'm not a diabetic but I do have Hypoglycemia (low blood sugar) and the diets are the same.
I stay away from all sauces (except really simple ones) and generally stick with beef or chicken dishes. You have to be careful even with things like the butter that is served because they sometimes have something added to enhance the flavor, such as honey or sugar. If I'm not sure of something on the table, I ask my DW if she can taste any sugar in it and if there's any doubt, I don't have it. If she isn't there to taste it, I just don't have it.
I have unsweetened ice tea with lemon at the meals and then I have coffee while the others have their desserts. Even if the waiter recommends something for dessert as 'no sugar', I don't eat it unless it makes good sense to me. Sometimes I think they mix up 'no sugar' with 'no added sugar', and it does make a difference. The menus say that they have 'sugar free desserts' every night in the dining rooms but they aren't always tasty, at least to me.
I really recommend that he fills out the medical form just for the benefit of having added help with screening the food at dinner times, it helped me considerably.
My biggest 'secret' method is to mentally imagine unkind things about the ones dining around me that are having multiple desserts every night, and especially the chocolate ones that even smell really good from three feet away. Just unkind things like a chocolate smudge under their chins for the rest of the cruise, or maybe some of that icing stuck in their ears or hair. Not kind, but it gets me through.
On the last cruise, a server in Palo's recommended that I try a non alcoholic beer that they served. I was very reluctant to try it because I was afraid it would have something else that would bother me, but he brought it to the table so we could check the label and it was Ok. I tried it and it taste's great, with no bad effects, it's called Sharps, and it's made by Millers.
Oh, I almost forgot, the Dessert Buffets are just plain EVIL, to someone on a diabetic diet.