You have a few options. You can rent a pontoon boat which holds up to 10 adults. Prices vary a little, based on whether or not you want the music piped in, but are in the $275 - $300 range. Snacks and soft drinks are included.
You can also rent the Grand 1, which is a yacht. I'm not sure of capacity (it's more than the pontoon boats). Price is around $450.
You can book these 90 days in advance by calling (407) 939-7529. They can pick you up at either the Grand Floridian, Contemporary, Wilderness Lodge or Polynesian.
We did this the evening of our wedding and it was great...a really special way to end the day.