Head West through the water again after you’re done with Clarke until you have to turn- then head North to the plank bridge.  Use this to head West and then South on the cobble to the edge of the sewer, where you can pick up an HP Up.  Come back to the bridge and hop in the water to the North, Surfing West again.  Reaching the end of this Surf area, turn North and hop onto the cobbled area so you can face a worker meandering around.

Battle: Worker Scott (680 Poke)
Opponent: Timburr Lv 17

Check the nook to the right and you can also pick up a Pearl, a nice item that sells for a good chunk of cash.

Return to the water and surf East to the end of the ‘block’, where you can hop onto land North.  Do so, and then hang a left as soon as you are on a wood bridge again, Surf-ing the sewer canal West and then North.  Jump out on the West side and you can dash South to pick up a hidden Ultra Ball.  Once you have that, head North as you otherwise will just wind up by Scott again.

Head East and you’ll soon reach the doorway that leads to a second scientist running experiments with the toxins.  Stop by for today’s medicine, then continue East until you can go North.  Head North as far as you can, going East when you have to, and you’ll come to a stairway that leads up into a back alley of Castelia.  Check to the West and you can pick up a Repel off the ground, then scamper North to find some musicians who aren’t too happy that you found their secret spot.

Battle: Roughneck Ricky (768 Poke)
Opponent: Scraggy Lv 32

Battle: Dancer Jean-Paul (1024 Poke)
Opponent: Darumaka Lv 32