Mike, I rarely gain weight on a cruise -- and I am one who has a rough time of losing weight at home. While I might indulge a couple of times on the cruise (escargot, chocolate mousse), I continue to be mindful of what, and how much, I eat. You plan sounds like a good one.
In many respects, I find it easier on a cruise as fruit and vegetables are plentiful, portion sizes are controlled (one of the reasons I use the dining room rather than the buffet), and exercise facilities are close by. In my case, I usually do a "walk-a-mile" on deck rather than using the gym, though. I also take the stairs a lot, and do a lot of walking in port.