I've made a little guide for you:
Start from Rustboro City, head north to Route 115. Follow the path until you reach the beach.
Use Surf and head west until you hit the border of the map (visible stones), simply surf north until you reach land.
This is how the area looks like.
Continue heading north, make your way past the trainers , until you reach the desert:
Turn right once you passed the stairs, battle the Hiker to turn south and try to grab the abandoned Pokemon. (Rock Smash required!)
You'll hit Meteor Falls B1F instead.
Good luck.