After you receive the Mega Ring and adopt Korrina's Lucario atop the Tower of Mastery, you go east to Route 12.
Get a Lapras
The guy in green near the Route 12 sign will give you a Lapras for free. If you don't have a space free in your party, go to the Shalour City Pokémon Center and deposit a pokémon into the PC Box, then come back and talk to the guy again to get the Lapras.
You will need to use Surf to get through this route, so you might want to teach Surf to the Lapras that you just got. Or you can teach it to another of your pokémon. It's your choice!
Battle and Catch
If you didn't already, be sure to go south and use Cut on the prickly tree to reach the Aspear Berry tree, and also go north into the fenced-in area and use your Dowsing Machine to find a hidden Honey in the corner.
Now go east and down the stairs. Teach Surf to a pokémon if you haven't already (the Lapras that you just got can learn it) and use Surf to get into the water. In the water, you may encounter wild Tentacool, Mantyke, or Lapras.
Swimming around in the water is Swimmer Alessandro. He has a level 29 Qwilfish and a level 27 Binacle.
Now go east onto the beach. You find Backpacker Joren on the beach. He has a level 28 Linoone.
Go south along the beach and use the Dowsing Machine to find a Net Ball on a rock.
Go east and up the stairs. The entrance to Baa de Mer Ranch is here. You can go in if you want. But before you do, go through the flowers to the south, then go down the slope to the west. Use Cut to cut down the prickly tree, then get the Leftovers to the west.
Now go back up and into Baa de Mer Ranch.
Baa de Mer Ranch
Go into the house to the northwest in Baa de Mer Ranch. The girl in the house will give you TM45 Attract.
Go outside and into the area to the right where there are some Skiddos. You can hop onto any of the Skiddos that are awake. Just walk onto it from the side. If you want to get off, press B.
Skiddo can hop over ramps, so go east and hop over the ramp. Go south from there, then go west over another ramp. Get off of the Skiddo and go north to the narrow path that goes west. Get the Whipped Dream that is at the end of this path. If you trade a Swirlix that is holding a Whipped Dream, it evolves into Slurpuff.
Go back over the ramp to the east, then go north and hop over the three ramps to the northeast. Get off of Skiddo and go up the small set of stairs to get a Shiny Stone.
Jump west over those three ramps again, and this time go south, then southeast. Notice that the fence is broken in the southeast corner. Have Skiddo jump over it.
Continue on Route 12
While riding Skiddo, you can pass by trainers without having to battle them. This page will list the trainers on the route, but you can skip them if you prefer.
You can hop off of Skiddo and walk west through the flowers to find Pokémon Breeder Amala. She has a level 27 Seviper and a level 29 Miltank.
To the east, you encounter Youngster Aidan. He has a level 27 Yanma, a level 27 Mothim, and a level 27 Whirlipede.
Keep going east, then north. The trainer there is Pokémon Breeder Foster. He has a level 27 Zangoose and a level 29 Tauros.
There are stairs north of the Pokémon Breeder, and Skiddo can't go up stairs, so you'll have to get off of Skiddo now by pressing B. Go up the stairs, then go west down the stairs. Use the Dowsing Machine to find an Ice Heal at the end of the beach.
Standing on this section of beach is Fisherman Murray. All six of his pokémon are level 24 Magikarp.
Surf north in the water, then west when you can. You'll find some shallow water with a rock in the middle. There is a hidden Water Stone on the rock.
You can optionally Surf north to reach Azure Bay.
Otherwise, go east to reach Coumarine City.