The mass market lines like Celebrity, RCI and Carnival follow the rules of the ICCL (I think that's the right acronym) which sets it's rules and odds payouts at Las Vegas and United Kingdom standards.
In Las Vegas, for instance, the slot machines are set for a maximum house advantage of 17% which means for every $100 put into the machine the player can expect a minimum of $83 dollars back. The casino always has the option to dial back their advantage to less than that if they wish.
Blackjack, roulette, and craps all have their odds set by the randomness of the playing apparatae.
BTW, the only difference is the drinks in the casino on ships are bought, not free.