The method was -
Wife and I eat exclusively Keto, long term. Since June 2018. So, no carbs, including no fruit, also no alcohol - not an issue for us because neither of us enjoys it.
We usually skipped breakfast, having only coffee with half and half. At home, it’s heavy cream!
For lunch, we went to buffet and I had endless salads with ranch dressing. Would have had blue cheese dressing but not available at the buffet. Also found some sautéed veggies offered daily. And a small amount of protein, like a chicken thigh or similar.
For dinner, we ate in the main dining room. Had a choice of about 4-6 entrees. Also appetizers, entrees, and desserts. We always chose well, and had them omit the potatoes and rice. We often ordered the steak. And always skipped the desserts. The waiters could not believe that part!
Our in-laws, on the same cruise with us, ate loads of cookies and ice cream, and most enjoyed plenty alcohol.