I don't think there is anything wrong with it, really. I mean, once your kids go to school full time, they are away from parents for 6+ hours every day.
But, in a situation like a cruise, I couldn't personally do that. There are too many variables there that could cause problems. My biggest problem with that scenario is that there is NO WAY for the parents to be contacted in an emergency situation and that is the #1 reason why I wouldn't leave my kids on the ship while I went ashore.
Any other time my kids are away from us, there is a way to reach an "emergency contact". On a ship, once you leave the ship at port, you are incommunicado and that makes me VERY uncomfortable. Heck, we had a situation where this happened while we were on the ship and the kids were in the club. The wave phones were not working, one of our kids was hysterical and needed us, and he remained like that for close to an hour before I showed up to get him. I had no idea and it killed me to know how long he was freaking out like that.
I would probably be comfortable doing it once the kids are WAY older, like over 16.