If you're going on the Dream the party will be on Deck 11, and to watch from above you'd go to 12.
And as you pull away from Port Canaveral, you might enjoy heading over to the starboard railing. DCL cast members usually come out of the terminal to wave good bye with Mickey hands! Just a bit further up the canal, look for the people at restaurants like Grills and Fishlips waving at you with Mickey hands, too!