Move on through Floccesy Town to Route 20 and then pass Floccesy Ranch and head into the river at the end of the Route.  The Dousing Machine will turn up a Heart Scale on the East bank, and continuing South brings you to an extra area.  There are two more item balls here in a similar place to the one back on Western Route 20, so you can reach those during the Fall.  The pokemon in the grass here are the same as on the rest of Route 20, but they're higher level- around level 10.

Make your way to Virbank now, because the ability to Surf opens up a new treat here.  Head down into the canals and you can cross the water to reach a spot you couldn't before.  The Northmost canal branch, reachable on the West side of the city, has a stairway that leads up to a Rare Candy.

Next is Castelia City, whose sewers just got a lot more interesting.

Return to the sewers from Thumb Pier, and follow the South wall as far as it goes, then head South.  Keep this up until you pass the running janitor.  South of him, you’ll find a small staircase that descends into the water- use this to begin Surfing and head South, then turn West when you’re blocked by the terrain.  Use the South shore to disembark and keep going West until you reach more water.  Hop in and continue South.  You’re soon blocked again, so once more take to the land and head West until you reach the larger floor area, where a hidden Poison Gem is waiting for you.

That snagged, head South and hop into the water yet again.  This time, though, Surf to the East until you can turn North.  Hop left behind the railing onto dry land and grab TM41 Torment out of its little nook.  Torment is a strange move that relies on your predicting that your opponent is going to try to either apply status effects directly or use stat- altering moves- when a pokemon is hit with Torment it stops being able to use the same move on consecutive turns for 3-5 turns.  This can really mess with your opponent’s strategies, particularly if they only have one move to use that’s supereffective against whatever’s Torment-ing them.

Go East again and hop onto the platform at the edge of the water so you can face down the scientist meandering around.

Battle: Scientist Clarke (816 Poke)
Opponent: Magnemite Lv 17