To tell you the truth, it's massively less that I would have ever assumed. There are around 20 million passengers a year on cruise ships. I counted 41 deaths in 2016, 95 in 2015, and 31 in 2014. And, this includes deaths in off-ship excursions as well, as well as crew deaths. 8 died in 2015 in a plane crash. And, 2015 had 23 suicides listed (must have been a bad year).

Just as an exercise, assume the average cruise is one week. That would mean that there are around 384,000 passengers cruising in any one week (I'm sure someone knows better numbers than this). Think of this as if it were a medium-size town. The 2016 number would be 10.6 deaths per 100,000 for that town. The US in 2016 had a death rate of 824 per 100,000. The US national rate for suicides alone is 13 deaths per 100,000. As for the murder rate, that site lists 18 murders going back to 1909 (I assume they are ignoring acts of war)

Cruising is not my thing, but it looks like your chance of dying on a cruise if pretty minuscule.