If you make the initial deposit and all payments with the Disney Visa, you can get a $50 beverage credit to use onboard. Actually, that offer is currently only for 2015 cruises but does typically renew each year so I expect it to be available for 2016 as well. You can use your Reward Points onboard to pay for your onboard charges; how much you have for reward points depends on how much you charge to the card prior to your cruise. There is nothing else "onboard" perk-wise -- no discounts, etc. You do get 0% financing for 6 months for any payments made towards a cruise.
If you get the Premier card ($49 annual fee) you can also use the reward points towards airfare.
Enjoy your cruise!