Hi there,
Onboard investigator is a lot like police investigator, but invested heavily in daily security routines, spending 90% of his time at gangway control, monitoring security staff performing their routines.
Once onboard, you don't go anywhere for any reason, for the next 4 months at least.
Carnival Cruises has
Brand Standard Inspector, which might be more in line of what you are looking for.
It includes lots of travel between ships, lots of free time, and as for money, it largely depends on where you're from. US citizen gets paid better than East European, Indian, Asian ...
Also, as US citizen, you would have to fight for a job, since Carnival likes to employ candidates from underdeveloped countries who can do much more with 100 dollars bill than US citizen can.