I have only been on 5 cruises in recent years with Carnival and Princess but have noticed that both lines employ a lot of ex Indian and Gurkha soldiers as security. On my last cruise, last year Pacific Princess the chief security officer was a retired Royal Canadian Mounted Police Officer around 60 and the 6 others were ex indian/gurkha soldiers in their 30's. As a retired cop I got to speak informally to the security officers as I was interested in their role. No doubt the junior security officers are NOT well paid like others in the ships company, eg wait/cabin staff.
Many years ago in Australia anyway police officers on annual leave (vacation) performed ships security duties in particular on the Russian/Italian cruise ships.
I would think some experience in law enforcement or the security industry would be essential for ships security officers. Hope this advice is helpful. 8)