Bring lots of hand sanitizer; cruises are notorious for breakouts of things like Noro-virus.
Bring a couple of flashlights for night time emergencies.
I would also bring some type of warm clothing like a watchcap and windbreaker; have it ready in case you have to abandon ship at night.
Last cruise I went on knives were hard to get through security. I brought a modest SAK in my checked bags. I don't think they will let you carry a knife on in your pocket. You do go through metal detectors.