6th Gen ORAS Question

Can I go to Dewford City and leave the Island to go back to Petalburg before I deliver the message to Steven and before I get the badge? I ask because I'll be heading there in a bit and want to grab a Makuhita and Magikarp for my party and train them up on some trainers I avoided to get exp faster for them.

I can tell you that the answer is yes. You can go back to Route 104 by talking to Mr. Briney.