I hope you have a great cruise and the weather holds! It's good that you already know that you'll be off your plan so be prepared. As I said, it would be a good idea to get some activity in to help counteract the food you know you'll be eating. Dancing! Walking the deck! Walking around when you're in port.
I'm glad you're concerned about
your blood sugar so it might be a good time to talk dosages over with your doctor, not just for the cruise but for general maintenance.
My thoughts are that there are times when I'm going to be off my plan and I know it, so I might be more restrictive beforehand and afterwards. However, I do know that I will always go back to my very low carb way of eating because that is what's best for me.
Bon voyage!