There is no exact answer to your question as different crew members are paid in different ways depending on their job and which cruiseline. Most of those in the service sector jobs make most or all their income from your gratuities. That obviously does not extend to the Captain, Cruise Director, First Mate, etc. Some cruiselines withhold a portion of gratuities and some pass the entire sum to those performing services for you. Some lines combine all tips and distribute them evenly among the staff. With many of the new freestyle dining options now available that's really the only way they can. You're served onboard by many, many different waiters, and other dining room staff during your cruise.
Some lines also have generous bonus structures in place for those crew members who consistently get very high ratings from passenger on those silly surveys they ask you to fill in and turn in. These are also often used for who will be promoted. Anytime you have exceptional service anywhere in a service oriented industry, the survey card may be of far more value to the person you're tipping than any monetary reward you may give. But please, still give 'em some cash as a thank you.
I don't think you have to worry about cruising being a source for oppression. The staff of these ships are generally in a much better financial position than many of their extended families and countrymen, at least in many of the countries they're predominantly from. Those who choose to be crew members from more affluent areas of the world do so for the experience to travel, the social oppt'y with the rest of the crew and other reasons. Nobody is held onboard against their will or forced to take these positions. They are sought after oppty's for many. One of my favorite waiters ever on a cruiseship (a Filipino) told me his brother had been trying fror over a year to get a cruiseship job and would continue until he was accepted. He was working hard on his english written and verbal skills, the reason he was initially given for being denied the job.