From Jubilife City, head west onto Route 218. You'll need to use Surf to cross the water. This route has a few trainers and wild Pokémon like Gastrodon and Finneon. Continue west, and you'll soon arrive in Canalave City.
Canalave City is a port town with a library and a steel bridge. Heal your Pokémon. You'll immediately encounter Barry, who will challenge you to a battle. His team will be stronger now, so be prepared. After defeating him, he'll tell you to check out the library. Explore the city, and then head to the Canalave Gym.