We did. No discount or honeymoon package or anything like that. There are certainly romantic things you can add to your experience on the cruise but they won't be at any kind of a discount nor any kind of special offer because you were married at Disney. Like the PP mentioned, you can often get onboard credit from booking through a
travel agent, so that's free money there - additionally if you have a Disney Visa and you pay the cruise entirely, every payment on the Disney Visa you get a $50 on board credit that can be used for any non-included beverages (so alcohol, specialty coffees and smoothies).
Some added extras we thought made our cruise extra romantic: booking a verandah room (if you're able), carrying on champagne and drinking it on the verandah on sea days, booking a Rainforest Room 1-day pass (cheaper than other spa services, ours was about $16/pp, for the Rainforest Room which is fancy showers, saunas, hot tile loungers and hot tubs), booking Palo Brunch (now $30/pp). You could also book Remy if you're on the Dream or Fantasy (more expensive), full spa services or in-room delivery packages, like robes and champagne or chocolates and rose petals. We also preferred having our own table at dinner, after 9 days of entertaining family and friends at WDW we really appreciated the alone time at meals.