where do spongebob and patrick go in the movie
Shell City.
The SpongeBob SquarePants Movie
 SpongeBob and Patrick team up to retrieve the crown from Shell City to save Mr. Krabs from Neptune's wrath and the oceanic world from Plankton's rule.
wiki:6532302
The SpongeBob SquarePants Movie [SEP] Introduction
Krabs for the crime. SpongeBob and Patrick team up to retrieve the crown from Shell City to save Mr. Krabs from Neptune's wrath and the oceanic world from Plankton's rule. Hillenburg accepted an offer for a film adaptation of "SpongeBob SquarePants" from Paramount Pictures in 2002, after having turned it down multiple times the previous year. He assembled a team from the show's writing staff, including Paul Tibbitt, Derek Drymon, Aaron Springer, Kent Osborne, and Tim Hill, and they structured the film as a mythical hero's journey that would bring SpongeBob and Patrick to the surface. The film was originally intended to serve as the series finale, but Nickelodeon ordered more episodes of the series as it had become increasingly profitable, so Hillenburg resigned as showrunner, with Tibbitt taking his place.