Route 21

Route 21
Surfing Types Level(s) Rarity
Frillish Frillish Water / Ghost Lv. 30 ~ 45 65%
Mantyke Mantyke Water / Flying Lv. 30 ~ 45 30%
Remoraid Remoraid Water Lv. 35 ~ 45 5%
Surfing (in surfacing fish) Level(s) Rarity
Jellicent Jellicent Water / Ghost Lv. 30 ~ 45 60%
Alomomola Alomomola Water Lv. 30 ~ 45 35%
Mantine Mantine Water / Flying Lv. 35 ~ 45 5%

Right near the start of the route is a Swimmer ♀ with a Wailord L46 on her team. Head over to the southwest of her and you'll find another Swimmer ♀, this one using two Remoraid, each at L45. There's a lot of rocks around, but swimming over to the west will find you a Swimmer ♂, and he's got an Octillery L46.

In between the male Swimmer and the female one, you can surf south to find a patch of shallow water with a Big Pearl on it. South of that is a Black Belt on the shore. He uses a Crustle L47. Go back to that one male Swimmer, but head east and then south instead, where you'll find another Swimmer ♂. He uses two Wingull L45.

Head south until you find a shore, and then make your west. You'll see a Doctor outside of the entrance to the Seaside Cave. He'll be very useful at healing up your Pokemon, but you've gotta beat him first. He has an Alomomola L46. Before you enter the cave, cut through the trees just to the west and fight the Fisherman, who uses three Frillish, all at L44 (the first is a male while the second two are female). There's a PP Up just past him.

Make sure your Pokemon are healed, then enter the Seaside Cave!