I know this is an old thread but I just got back from WDW and went on this so I thought I would share my experience.
I had no idea this existed until we were walking by and saw the "reservations available" sign. Initially when we heard the price, we were going to pass (it was my mom and myself, $50 each, my 6 year old who was $30 - my littlest was free). But this was a pretty inexpensive trip for us (yay for cashing in old park hoppers that never expired!) so we decided to go for it. My mom loves parades but we never watch them because I hate standing in crowds. And we've never met Tiana or Naveen before.
I haven't read through all the past posts so I don't know if this has been covered but I'll tell you how our experience went. You first walk into the "holding area" where they have ice cream with a bunch of toppings and small cupcakes and pastries. Tiana and Naveen are there taking posed photos. Then they leave to go do the parade, and you board the boat. The boat pulls forward from the dock only slightly and that's where you watch the parade from. I saw some comments that the viewing was not good - it was great for us. We saw the parade as it passed the Diamond Horseshoe and could see everything. Many of the characters waved right to the boat as they passed, which was fun.
There are unlimited snacks on the boat the entire time - ice cream bars (Mickey, frozen bananas, ice cream sandwiches and fruit bars), bags of candied popcorn in mardi gras colors, and salted popcorn, bags of pretzels, cotton candy, and water bottles and soda cans. You can really take as much of everything as you want - and everyone stocked up on snacks to go before gettting off the boat too (we joked it was our mission to eat and/or take $50 each in snacks and drinks - with park prices for these things what they are, I honestly think we came close!). We almost ate our admission fee in Mickey ice cream pops alone!
After the parade, the boat goes back to the dock, picks up Tiana and Naveen and then goes around the river. Cast members hand out mardi gras beads and instruments to the kids, and they have a parade around the boat and dance party with the characters. Then it's back to the dock and it's all done.
It was our favorite part of our trip this time, and we had some pretty epic moments this trip! As my 6 year old was chowing down on a cup of gummy bears she said "this is TOTALLY worth it"
I would say there were only about 15 families on our boat, which meant that the kids really got a lot of attention from the characters. Tiana was super sweet and Naveen was goofy and fun. We will definitely be doing this again.