We too homeschool. When we're on vacation, we're on vacation. This teacher needs a break every once in a while too! That said, I encourage the kids to research where we're visiting before we go so that they have a better appreciation of what they're seeing while we're there. And when we return I often have them write about their adventure. I also encourage them to keep a journal while on vacation, but it's not mandatory.
We took our first cruise last fall and are taking our second cruise this coming fall. Because cruising was new to us, I also encouraged the kids to read about cruise ships, ships in general, etc, before we sailed. While on the cruise I think the kids learned a lot on their own. The cast members are so friendly. The kids struck up so many conversations and learned a lot from those conversations. They came home with questions and things they wanted to know more about so we researched some more. My one son was so intrigued with the mind teasers and the magic tricks that our assistant server showed him that he came home and learned more so that he could "impress" family and friends. My daughter met another homeschooler and they've become pen-pals.
We saw taking a cruise as an opportunity for other kinds of learning opportunities. Hopefully others will appreciate this, maybe we're just very sheltering parents. But for us, one of the struggles with homeschooling is giving the kids enough opportunities for independence. The kids learn at home, they go to camps, they participate in clubs and coops, etc, but there are always parents around. Being on the ship, the kids had a fairly large, yet contained space to explore - on their own! You're hungry? Go up to deck 11 and get some food... By yourself. You want to go hang out at the Edge? Go, you know you're way to get there. Again, maybe it sounds odd, but the kids gained a lot of confidence and independence on our cruise.