Blackjack tables on most ships begin at $5. I don't play roulette onboard the ships as thet only give you a 1 to 1 spread on RCCL/Celeb (there is usually one 2/1 night) and 2 to 1 on Carnival and many others. They really stack the odds against you on a ship.
There is often a single deck blackjack table, I like these best.
good luck, Neil