IMO after several Cruises:
Small Flashlight.
Your Favorite energy Bars, a few to eat a few to save.
A pk or two of gum.
A water bottle of your own.
A sharp thing in your toilet bag.
Sewing kit.
A small waterproof neck safe stocked with some sort of ID, CC, Cash.

Truth is a Cruise Ship will provide most anything you need if you know
where to find it.
In a real emergency the mini bar or any bar I am near will be raided for water and snacks.
A light will be handy for the dark passageways, a headlamp more so
but that will not fit in your pocket for daily carry.
I will not carry my PassPort in Port unless I am required to, I do have my
D/L, Ship ID, and CC.
I use the neck safe when ashore, normally not while on board.
My Cell Phone remains on board in the safe with PassPort.

Unless you are a ditz if you miss the boat its your fault.
If the company puts you ashore you will have an opportunity
to pack and take at least one bag.
In your normal carry on you should always have basics.
Note book, wallet & ID, Swim Shorts, sandals or shores.
Meds and script, toothbrush. IE basics in case your bags are
late. I was on a cruise last year to Alaska and over 60 people
never Got there bags, the bags were never loaded on the boat.