I've put a Ferry Terminal (with Cruise Ship Dock) and a casino right next to it. Sadly, there seems to be very few tourists arriving through the ferry. Is there any way to increase the amount? The Bus Terminal at the other casino seems to bring in loads of tourists.
Your problem may be you don't have any connecting cities with ferry docks to bring in tourists from other cities. If this is the case I suspect this could be limiting you but cannot say 100%.
Here's some related information about casinos and tourists in general (VERY HELPFUL):