The prohibition I've heard about is that a cruise ship with a casino cannot begin and end a cruise in Hawaii even if the casino is closed.

I haven't cruised to Hawaii from the mainland but I have been on a repositioning cruise from Tokyo to Honolulu and the casino was open until we reached US waters. I would suspect that those cruise lines trying to entertain US tourists for 10-12 days cruising from the mainland to Hawaii also find a way to open their casino when on the high seas.

However, if you value your money you will avoid the casino on any cruise ship. There are too many better casinos to gamble in here in the USA. It's OK to pick up a few chips, though .