The casinos are on the ships are not "crooked"; they simply have odds that favor the "house" more than the casinos in Las Vegas. In other words, tighter slots, lower pay tables on video poker, more decks in the shoe, etc.
The casinos on the ships have a captive audience. It's the only place you can gamble. The casinos in Las Vegas know that if you aren't doing as well as you hoped, you might cash out and go to a different casino down the block. That's not an option on the ship.
Whenever possible, you should always be aware of the odds before you decide to wager. Of course, you can't do that on slot machines, which is why I never play them.