I know that 24 people have been reported missing from cruise ehips int he last 2-3 years. many have not been found.
Like any place where there are people, crime will happen. Just as you have to watch out for yourself on a land vacation, you also have to watch yourself on a cruise vacation. Those who drink, flick through wads of cash where others can see you, and wonder alone are prone to incident, whether it's their own fault or another passenger/crew member is involved.
Use common sense, don't drink, don't take medication that can make you sleepy, and don't go anywhere alone. Don't leave your kids alone. I'm going on a 4-day Bahamas in late July with my 14 year old son and he'll only be "alone" when he's in the bathroom. I love him enough to smother/protect him (and he knows that). If he wants to hang with other teens, it will be on a public desk where I can hang out nearby and read a book with an eye on him. No teen excursions for him. We'll explore Nassau and Freeport on an excursion together or the beach.