I think every life vest should have a whistle.

I think the knife / multitool ban depends on the cruise line, but I wouldn't try to sneak one on. You can pick up at least a butter knife once you're onboard if you absolutely can't live without it, but I wouldn't.

I think a flashlight (or one per person) is an excellent thing to carry. It's pretty useful just in day to day situations.

I'd consider carrying hand sanitizer and wipes. Some cruise lines like Disney bombard you with these but I think others just ask people to wash their hands. Ships get some nasty outbreaks and that might be your biggest realistic threat.

If the cruise line allows it, I like to pack a suitcase of bottled water. Or booze.

If your party splits up, it helps to establish some place to keep a little notepad to pass messages. Or you could try using the phone / voicemail system to do that. It's amazing how crippled we feel without cell phones. You could try some FRS / GMRS radios but if you don't use them right they're useless. A couple postit notes on a cabin door work fine.