Where do you go after beat the gym leader in dewford town sapphire?

After beating the Gym Leader in Dewford Town in Pokémon Sapphire, you should head to Slateport City. To get there, follow these steps:

1. Exit the Gym and go to the southern part of Dewford Town.

2. Talk to the sailor standing near the boat and select the "Take me to Slateport City" option.

3. The boat will take you to Slateport City.

4. Once you arrive in Slateport City, disembark from the boat and explore the city.