Many cruise ships have casinos on board, and since they spend a lot of time traveling through international waters, it's perfectly legal. However, that also means that they aren't subject to a single area's laws concerning gambling. So, the rules onboard will sometimes change depending on whatever country or city is nearest. For example, your odds of winning can change dramatically depending on where in the ocean you are.
Cruise ships might switch up the number of decks they use for blackjack, etc. In short, just be aware that the rules governing their games aren't set in stone, and they often change.