I've only seen UK retired navy personel in this position (those there are exceptions). I'm fairly certain the cruise lines hire from specific hiring partners or recruitment agencies - but you might try contacting the HR departments of the lines you want to work for to see if that's the case
this is one I found in one of those cruise employment books I bought years ago
Logbridge Ltd
South Western House
Canute Road,
Southampton SO14 3FG
http://www.columbia.com.cy/index.htm
just googled "maritime security cruise ship employment" and found this - at least it's a good description of the job albiet a pay site
http://www.nixers.com/ApplyForJob.as...Ships.Security
If you dive or have marine/mechanical abilities, you might want to look into private yachts and charters. Knew an ex military guy who found a job on a as security for high end clients who value their security/safety
good luck