Surf south of the P2 Lab and sneak through the currents to find a patch of shallow water. There's a hidden Shoal Shell in the northeast corner of that patch, plus there's a valuable Pearl String sitting there for you, too, which can be sold for $25,000.
Surf over to the west of the P2 Lab until you reach some more shallow water. There's some Shoal Salt hiding in the southern part of the shallow water. Pick that up, then head west, making sure you don't get caught up in the currents.
Fight the Swimmer♂, who uses a Carvanha L65 and a Sharpedo L65. Swim south of him, once again avoiding the currents, and then sneaking between the currents flowing east. There's just a one tile wide path you can take, but you'll need to take it west until you find a Swimmer♀. She has a Chinchou L65 and a Gorebyss L65. Take her down and then swim south, then west of her until you reach Route 18 again.
There's a Pearl hiding on the beach when you arrive, but before you land there, you should swim south to another beach area. There's a PP Max sitting out in the open for you, as well as another Prism Scale hiding just south of it by the rock.
Back on the northern beach, head west and fight the Battle Girl. She uses a Poliwrath L65, a Sawk L65, and a Medicham L65. North of her, you'll find a field of thick grass.
|Scrafty||/||Lv. 65 ~ 66||30%|
|Sawk||Lv. 65, 67|
|Throh||Lv. 65, 67|
There's an Iron sitting in the thick grass for you to pick up, plus there's also a hidden PP Max in the corner of the thick grass.
Alright, well that's actually all there is for you to do in Southern Unova! Now would be a great time for you to return to Reversal Mountain and fight against Heatran now that you have the Magma Stone. That will be covered in the Legendary Pokemon section of this walkthrough.