I can see both points of view here. What I don't understand is why some people seem to be arguing that only one way is correct.
School is a great place to learn. And while at school, children will learn more if they think it is fun, and worthwhile, and interesting. Learning just to get a grade is of little value, but learning how to learn and learning how to take a test and perform under pressure is very useful. Holding up under the rigors of a curriculum's requirements is very valuable, regardless of the specific courses being taught. What they learn may or may not be relevant later in life, but how they learn and their attitude towards learning will last forever.
Likewise, a vacation is a great place to learn. World perspectives, self-reliance, interaction with others, and again, the joy and process of learning. Some appear to say that since a vacation is fun it can't be learning, or that it shouldn't include learning, or that one type of learning has less value than another. (If you didn't say that, then I am not addressing you. No need to tell me that I am wrong about what you said.)
Of course, it is always up to the parents and the students to make up any work that is missed, and that for some students this will be harder than for others. It is also true that an absence is a burden on the teacher, no matter how much we try to minimize that, and their tolerance is to be greatly valued. But I think that a vacation, even during school time can be greatly valuable as well.
One of the ways our educational system shines is when it doesn't just teach facts and figures to be regurgitated, but when it teaches how to ask questions, and listen to others ideas and opinions, and how to think new thoughts. Cruises can be very helpful with all of these goals. More and more educational systems are understanding the value of this interactive style of learning and how it works best when children are willing participants. Math drills on deck 4? Absolutely. Facts have their place too. Periodic table with a soft serve cone. Certainly. Plus asking people where they are from and what they do for a living and listening closely and respectfully to their answers. And a thousand more things. Life, in and out of school, can be very educational. Or not. Our choice.