Found this in the Birnbaum guidebook for you. Disney's pontoon boats accommodates up to 12. They take guests on tours of the Seven Seas Lagoon & Bay Lake, near MK, as well as Crescent Lake, near Epcot WS. Those in MK area are treated to VIP viewing of that park's fireworks, while Epcot-area cruisers take in Illuminations. Cruises last 1 hour. MK fireworks excursions depart from GF, Poly, Contemp, Wilderness Lodge, & Ft Wilderness marinas. Illuminations cruises leave from Boardwalk & Yacht & Beach Club marinas. Cost including driver starts at about $150 per boatload.
Hope that helps. I wish I could tell you from personal experience! Have fun!
There's another one listed in the same book for a 45 ft Sea Ray yacht for 16 guests starts at $400 per hour with the per-boatload fee covering up to 14 guests, plus driver & deckhand.