Now that you have Surf, you can backtrack to reach a few new optional areas before moving onwards. To start, head back to Nuvema Town go up to Route 1. Surf to the west on the water and jump off at the southwest part onto dry land. West of that is a Max Ether. Go up the stairs to battle a Pokémon Ranger.
If you go northwest of him and into the house, the Ace Trainer inside will heal your Pokémon for you. The Pokémon Ranger will give you a Pokémon Egg. This Egg will eventually hatch into Larvesta. Make sure to have an empty space in your party to receive it.
Head across the bridge going south and use Strength to push the boulder into the hole. In the southeast corner is a Max Elixir. Head back across the bridge and go down the stairs. Go up and then south down the narrow path to pick up HM05 (Waterfall). This will allow you to climb up large waterfalls. Go back to the stairs and take the smaller set of stairs just below them and push the boulder into the east hole. Keep going east to battle another Backpacker.
Above him is a Dragon Scale. The only purpose of this is to evolve a Seadra into Kingdra by trading it while it holds it. They cannot currently be found in the wild, however. Also hidden in the rock nearby is a Calcium. Go north of where you picked up the HM for Waterfall and take the stairs up. You can battle a Veteran here.
Go below him, across the bridge, and down the stairs. You'll be back on Route 17.
Surf in the water and take the bottom current to the east side. Go between the currents north of the shallow water to the rock at the north wall to find a hidden Heart Scale. Head south between the currents and onto the shallow water. Use the Dowsing Machine to pick up a hidden Big Pearl. Head back up between the currents to where the current took you before and go east. You'll be in a place called the P2 Laboratory.
Go inside the building. The only item in the building is a Dubious Disc, which, like the Dragon Scale, is used to evolve a Pokémon not yet obtainable. Head back outside and into the grass to the south to battle a Scientist.
Go north of her and up the stairs. Head west to pick up TM19 (Telekinesis).
Surf east of Driftveil City's Pokémon Center to the left side of the lighthouse to pick up a Water Stone.
If you go to Route 6, west of Driftveil City, Surf just north of the bridge to grab TM84 (Poison Jab). Surf up and go east across the bridge, and keep heading north. Surf north, and on the east side, pick up the Silk Scarf in the corner. This will raise the power of Normal -type moves. Keep going west and travel until you get to the rest home. Go through the dark grass and go east. Jump out onto dry land just south of the river. You can use your Dowsing Machine nearby to pick up a TinyMushroom and a Big Mushroom. Surf east until just before the entrance to the cave. You'll find a hidden TinyMushroom nearby. Now you can enter the Mistralton Cave.
In the rock north of him is a hidden Hyper Potion. East of him, go down the steps to pick up a Max Repel. Go west and down the stairs to push the boulder into the hole. This'll give you a shortcut back to the entrance. Go back up the stairs and northwest to grab another Hyper Potion. Go south past the steps near the boulder and take the stairs up to the next floor.
Go down the stairs and go all the way down to grab a hidden Ultra Ball in a rock. Push the boulder into the hole, and go east to grab another hidden Hyper Potion in the rock. Head up to the platform, go east, then down the steps and push the next two boulders into the respective holes. Head back east and go up the steps. Go all the way west to pick up TM80 (Rock Slide). Go back down the stairs and take the narrow path below the raised platform to the west. Go down and pick up the Iron, and west a bit more for a hidden PP Up. Hop down the ledge and go up to the next floor.
You'll now be in the Guidance Chamber. Just to the east is an old man who will tell the story of the three Legendary PokémonCobalion, Terrakion, and Virizion. Take the path east of him to the north to grab a hidden Ultra Ball and a Dusk Stone. Go past the old man up the platform, then head off and push the two boulders into the holes. Just north of the boulders is a Rare Candy, with a hidden Carbos in the nearby rock. South of the second boulder is a Revive.
Now go up to the northwest part of the Guidance Chamber. Here, you'll find the first of the Swords of Justice trio, Cobalion.
After the battle, the old man will come up and tell you that since you managed to capture Cobalion, that Terrakion and Virizion will trust you as well, and will give you the opportunity to find and capture them. Terrakion won't be found for a while, but Virizion can be found in an area near Pinwheel Forest called Rumination Field.
Take the entrance from Nacrene City and go north. Wrap around to the southwest and Surf west to grab the SilverPowder, which raises the power of Bug -type moves. Go back east and take the northeast stairs north. Go through the hollow log and continue north to reach the entrance to the Rumination Field.
Waiting for you inside is the Legendary Pokémon, Virizion.
There are several other places you can backtrack to now that you have Surf.
On the west side of Striaton City, Surf across the pond in the northwest corner and talk to the man there, who will give you a Big Pearl. By Surfing north to the northeast corner, you can find a hidden Zinc.
Go north to Route 3 and go to the southern part near the bridge. Surf all the way north of the bridge to grab a Full Heal. Surf to the west onto the grass to pick up a Max Ether. Just up the stairs with the Dowsing Machine, you can find a hidden Rare Candy. Go north to battle a Pokémon Breeder.
Go into Wellspring Cave just west of Route 3. If you Surf to the small indent below the raised platform, you can find a hidden PP Up with the Dowsing Machine. Go up the stairs of the raised platform and, using the Dowsing Machine, in the bottom corner you can find a hidden Full Heal. Surf to the east side to grab an Elixir. Go south and east down the steps to grab an Escape Rope. Go east and down the stairs.
Head to the southeast section to find a hidden Max Potion. Head back and Surf to the east, then head south to pick up TM47 (Low Sweep). Head back and Surf north on the west pond. Surf east and then go south to the ground. Head up the stairs and take the southeast stairs down to grab a Mystic Water, which raises the power of Water -type moves. Go down the steps east of this for a hidden Revive. Go west to find a hidden Hyper Potion in a rock. Go down the stairs just south to battle some Trainers.
Head up the platform above them and go to the southeast corner to grab a Dive Ball. Surf north of the Fighting -type trainers and you can use the Dowsing Machine to find a hidden Max Elixir on the east side next to the wall. Go up the stairs to collect TM52 (Focus Blast).
If you go into the house north of the Pokémon Center, talk to the boy and he will see that you've defeated Skyla. He says their treasure was left at the end of the runway. Head to the southwest part of the runway to grab TM40 (Aerial Ace). You'll be approached by the little boy and girl from the house. They were kidding about it being their treasure and let you have it.