You can walk up to the boat launch dock (this is located near the restaurant Narocoosse's) or the promenade just outside Gasparilla Grill & Games near the marina area of the resort. From these locations, you'll be able to see the castle and the fireworks.
The beach is another idea but some of the lower fireworks will be blocked by the lodge building.
Yes! From the backof the GF,where the Gasparilla Grill and Games is located is an excellent spot for viewing the fireworks. You will be able to see the castle straight ahead from this location. Have Fun! Joan