I asked the same question as OP before our first cruise last February. We traveled with our daughters, then aged 13 and 10. We also gave some directions/rules beforehand, never really expecting that they'd hang out together. We were so wrong! I should mention that we'd been on vacation for a week before the cruise, so they'd spent A LOT of time with us leading up to boarding. When we first got on board, we explored the ship and had lunch. After lunch, we checked out our stateroom and while we were unpacking, our 13 year old said, "So, can [DD10] and I go?". What?? DD13: "We want to explore the ship!". And off they went! It was actually a nice bonding experience for them (as DD10 can also be an irksome thing to the oh-so-mature DD13).
Our main rules were that they needed to stick together, not eat too much ice cream, no going into anyone's room and remember your manners!
The kids were instructed to stay together and the only glitch we had was when DH and I went to dinner at Palo. The kids were going to have dinner with our friends from home who were also on the cruise. Well, DD10 needed to go to the room before dinner and was spending some time in the bathroom and then told DD13 she could go on without her and she'd catch up. Yep, broke the rule to stick together...but this was day 3 and DD13 thought DD10 knew what she was doing. So, DD10 finally leaves the room and gets completely turned around and lost. Somehow, a female cast member who was assisting a character realized that DD10 was lost (eventhough DD10 swears she never said anything) and the cast member figured out where DD10 was supposed to be (psychic? I'm not sure, DD10 also swears she didn't show her KTTW). I chalk it up to a little Disney Magic. For our next cruise, we will be requiring the kids to bring walkie talkies with them.