Adrian - can't believe you booked a cruise without giving a thought to tipping. If you were on a land-based vacation and ate three meals a day in a restaurant, wouldn't you be tipping for that? Probably quite a bit more than $3.50 per day each (that's little more than $1 per meal). Yes, for these people (cabin stewards, waiters, etc.) this really IS their salary - most get only about $50 per MONTH from the cruiseline and, like waiters and waitresses in restaurants, depend on these tips.
Some of the upscale cruise lines do include tips in the price of the cruise - BUT, you probably couldn't afford them.