A good starting point to find a job at a cruise liner is this job
board: http://2ajobguide.com/cruise_line_jobs.aspx
There I found over 10,000 cruise employment opportunities published in
the USA - even though not all of these are really related to on-board
cruise liner jobs (for instance you'll find a job vacancy for an
exploration leader at Cruise West in Washington, just to name one).

Anyhow, IMO a powerful free job board which enables to apply directly
online or to subscribe to an email alert list for jobs in your region,
for instance.