Firstly, just wanted to say that I am enjoying this thread and it is good to get suggestions around behaviour that I may not have considered.
I just wanted to make people aware about reprimanding other people's children if they do not approve of their behaviour - on a cruise or otherwise.
My son has aspergers. If you looked at him you would have no idea that he has this disabilty. His behaviour can at times be challenging and may cause adults to look at him and label him a 'naughty boy', which he is most certainly not. As I am aware of these issues, I make a point of supervising him as much as is humanly possible. However, I also have a toddler and I simply cannot do it all!
Comments/looks made by strangers (although maybe done with the best intentions) do not help and actually increase a very stressful situation. So please no 'teacher' looks or comments made - please think that you have no idea if a child has such a disabilty!
In the situation outlined before, the young boys weren't being supervised by any adults, so in my opinion, the "it takes a village" approach of adults reminding children what appropriate behavior is seems totally appropriate, whether one of them has a hidden disability or not. If a child has a hidden disability such as yours, then I doubt you'd allow him to head out with a bunch of other kids alone, and if you did believe he was ready for that, then it's reasonable to believe that other adults would call the kids on their behavior if they were getting rambunctious. Kids need to learn how to behave appropriately even when not supervised - I don't necessarily think 100% constant parent-over-shoulder is the way for them to learn that - but I appreciate knowing that other adults will remind kids that running in hallways, or yelling/screaming in the hallways, or any other host of typical stuff that kids-in-packs do when they get exuberant - needs to be dialed down a notch.
That said - if a child is acting up and the parents are right there - even if they aren't or can't deal with it at that time (as you reference there are times when no parent can do it all!), then that's another story. I'm more prone to step in and comment to children who are by themselves and will leave the parenting to the parents when they are there and just overwhelmed.....been there, done that, and totally agree that sometimes I appreciate the patience of other adults when I was having to deal with one and not the other!
Rounding back to the OPs question - I do see a lot of parents-helping-parents on cruises, from that "ettiquette" question, especially around the pool. I've seen adults helping kids stand up after they slip/fall, pointing out where the drinks are, helping kids reach the cups, reminding them not to re-use the cups, you-name-it.