Hmmm.....I have sailed NCL, Carnival, MSC, Celebrity, HAL and Royal...on multiple ships. All had a casino. All the casinos were decent sized.
So, to answer the OP's question, YES, part of it is a floating casino. The ships are also floating buffets (which are also sizable on cruise ships). There are also floating sun decks, and floating theaters, and many floating bars, etc.
There are lots of things to do on a cruise ship. Casino being just one of them.
I think Disney cruises do not have a casino. Might want to check into their cruises.
Overall, Europe has a higher smoking percentage of the populace than does the U.S. So, given the OP is from Europe, their cruises will likely have more smoking than those that sail from the U.S.