Re: Adding an upper deck to a pontoon....
You could try this place:
http://www.u-fabboats.com/custom/party boat.htm
Or try a google search for other options. There a lot of boats around here with upper decks. People who say you raise the COG 6' don't know what they are talking about. All the weight of toon is in the toons, the fuel tanks, and the motor. these are not prone to rollover, and with all of the decked toons around here (there are dozens if not hundreds) I have never heard of a rollover and I my firend in the FWC has never heard of a local rollover.
The things to be aware of here are the local regulations regarding the operation of a boat with an upper deck. I think that the upper deck cannot be occupied while underway, and the weight distribution of the boat is that same with any boat in that the captain is responcible for the boat, the passengers and the load, so use your head or the water cops can ticket you for wreckless operation.
Have fun with your build! Last weekend we saw a 20' toon with a nice upper deck and a slide for the kiddies. The slide used a bilge pump to pump water to the top of the slide. That kept the children busy all afternoon, while two of the mothers sun bathed on the top.