Alcohol is a major contributing factor of personal safety incidents on cruises, so go easy on the booze.
If you're a solo cruiser, find a friend early on. That way he/she can look out for you.
If the door has a dead bolt, use it. If it doesn't, consider a doorstopper. Cabin stewards carry plenty, so ask for one, or bring your own.
This is a tough one, as there is nothing like listening to the gentle lap of the sea against the hull while you sleep, but for safety's sake, keep the door locked at night.
The vast majority of cabin stewards are honest, but it's not worth putting temptation in their way.
Ask his or her name on day one. Establish a rapport. He or she will notice if someone other than you is trying to get into your room.
When on shore excursions, take out what you need, but don't advertise it. Keep your cash in a money belt attached to your body.
This is not a good idea, ever. It could result in instant dismissal for the crew, and you will likely be asked to leave at the next port stop.
Before your vacation gets started, you must attend the muster drill.