In case you accidentally jump off the ledge and end up in Rustboro City, and think that you are stuck,
- You can go east & go to Rusturf tunnel.
- Use rock smash and proceed to Vendanturf town.
- Continue till Mauville and finally you can climb up the Mt. Chimney and proceed with the storyline.
One possibility is that Mr. Briney is at Slateport City, and you haven't asked him to sail back to Petalburg Town.