San007 is allowed his or her theories, just as everyone is. You have yours, he or she has his/her's, and I have mine. And who's to say with absolute certainty that San007's isn't correct? After all it is possible.
But just because something is possible doesn't mean it is probable.
San007, your theory is possible, but highly unlikely. Some sociopath could be hopping from ship to ship committing random murders for his twisted pleasure. But most murders are committed by someone with something to gain by them. Bump the husband off to gain the insurance settlement. Bump the cheating wife off in a jealous rage. One doesn't wander the decks of cruise ships and toss passengers overboard just for fun. The cost/benefit ratio isn't in his favor.
The terrorist is going to make the same cost/benefit calculation. Why toss one passenger overboard when he can walk into the dining room, blow himself up, and kill dozens of passengers? (Assuming he could smuggle the explosives on board.)
But being aware of your surroundings is always good advice.
At the same time, the probability of such an incident is so extremely low.
So go on your cruise. Eat drink and be merry. Each day is a gift. Not to be feared, but to be enjoyed.