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Before leaving Petalburg you can additionally reel in a Corphish with that good rod in the water there. It's in the same boat as Carvanha here, being gipped a good bit by its typing this generation. It evolves into Crawdaunt at level 30. Its shell may be based on batesian mimicry in association with that of Sharpedo.
South of the southwest pond in Petalburg you can fish up a Rare Candy.
You can now surf on the series of routes between Briney's cottage and Slateport. For now they're mostly good for experience. On a rock southeast of the Abandoned Ship (north of route 108) is a Rare Candy to snag (By the way, forgive me if I don't cover every single Rare Candy location in this thread. There are compilation videos on YouTube for that if you want them!).
Getting to go inside this ship is so cool. There are tons of rooms to explore and you can even pick up a TM for Ice Beam here right now. We'll need the HM Dive to get the real goodies inside of here later however.
Lastly for now off of Route 115 north of Rustboro is some Surf connections leading up to some tall grass where you can capture Jigglypuff, something likely overlooked to the less hardcore player!