We have yet to run into those tipping, 'scams' mentioned by Paul on a cruise, but we had an Amtrak room attendant once try something similar. He was so shameless and blatant, thinking we were fools, that needless to say, he didn't get a tip!
We always tip on board ship. The staff and crew work long hard hours. Most cruise employees have a strong work ethic and appreciate the job they have. And, I believe they do earn a decent wage.
We tend to think that 'service' jobs are menial, somehow degrading, and so any wage they earn would not be enough to cover the embarrassment of having to serve and clean up after someone else. Actually, some of the staff and crew on cruise ships LIKE their jobs...and want to KEEP them. They feel lucky to have them...enjoy the travel and meeting new people. When they've had enough, they quit. Yes, and some when their contract is up, sign on again!
I don't by any stretch of the imagination believe they are indentured servants or slaves force to work at a job they hate or that they are grossly underpaid.
We tip because we appreciate the good service and efforts they make to ensure our cruise is special and enjoyable.