Yes, but I wouldn't recommend it to anyone. Way too difficult. That said, I just got back from my 7 day cruise to the Caribbean, and not only stayed OP 100%, but lost 2.5 pounds on top of it. This was easily my biggest challenge on IP yet. I switched to phase 2 for the week. We had the same waiter and assistant waiter each night for dinner and by the end of the cruise they knew IP almost as well as I did. They knew not to offer me the variety of dinner rolls each night. They knew I needed my veggies steamed, no oil or butter and even which veggies I could and couldn't have, and to never offer me the dessert menu each night to look at. It was like having my own personal coaches there each day. I brought my bars for breakfast every morning and I hit the buffet for lunch each day. I even brought RTD for the times I needed some protein when I was on a land excursion and was starting to get hungry.
Even as my mom (who isn't on IP) would sit there at dessert eating her warm chocolate cake dripping with some yummy looking sauce and telling me how proud she was of me for sticking with this on the cruise, I hung in there. All I had to do was look around at all these really overweight people stuffing their faces with food, and that was enough motivation for me. Almost forgot, also wearing my much smaller new clothes also really helped too. I also hit the incredible gym and walking/running deck they had on board most days too. Nothing better then walking on that deck watching the beautiful blue ocean go bye.
So can you cruise or vacation while on IP? You bet you can. Would I recommend it, probably not. Lets just say I was one happy camper last night to sit and enjoy my vanilla pudding shake again at home. Tomorrow is my WI day and I'm going to have one happy coach.