Pokémon FireRed and LeafGreen/Route 19

'''Route 19 is south of Fuchsia City (where the Safari Zone is). Its south end is at Route 20 east of the Seafoam Islands. It features many trainers, all on water. You should swim with a repel on, as the waters will bug you repeatedly with Tentacools.'''

To reach Cinnabar Island, it's possible to fly to Pallet town and take Route 21 instead. However, that would skip the Seafoam Islands for now.