I would imagine you'd contact the resort marina in any of the ones situated around the Seven Seas Lagoon for reservation and pricing information. Give some thought to what you would do if the weather was bad and the fireworks were cancelled or delayed??? Options: Why not score a spot outside The Plaza at the ice cream tables? That is a great place to watch not only the show but see Tinker Bell go right "overhead"
There's so many seating spots you could find in all directions from the Castle...just walk around and scout them out....be prepared to get there early...and hakuna matata (no worries)