Stormalong Bay at the Yacht & Beach Clubs, home of the DVC Beach Club Villas resort. This is currently the only resort pool on property with a lazy river. It's small, and often too deep to stand up in (unlike most lazy rivers at water parks) but it's a nice place to relax on a tube for a while.
They are putting a new large-scale water area into the Kidani Village section of Animal Kingdom Villas, but I don't believe there will be a lazy river.
Amanda