Depending on itinerary:
Copy of passport or passport. (not in backpack)
Perhaps rain gear
Daily ships newsletter with name, address and phone number of ship's agent.
Small first aid kit with band-aids.
Credit card (not in backpack)
Enough cash that it won't be a major problem if lost. Divided between wife and I.
A lot has to do with what we'll be doing and where we are. Sometimes the backpack stays on the ship or is loaded with more or less "stuff".
The key is to not take too much and feel like a pack mule. Credit card, copy of passport, driver's license, cash and ship's card are the only requirements.