After defeating the Team Magma/Aqua Admin in the Team Magma/Aqua Hideout in Lilycove City, you can Surf east to Route 124.
Explore the Route
Go east and you can battle Swimmer Jenny, who has a level 39 Luvdisc.
Southeast from her, you can have a double battle with Sis and Bro Rita and Sam, who have a level 39 Pelipper and a level 38 Whiscash.
East of the Sis and Bro, you can battle Swimmer Spencer, who has a level 37 Staryu and a level 37 Tentacruel.
North of Spencer, you can battle Swimmer Roland, who has a level 39 Sealeo.
East of Roland, you can battel Swimmer Grace, who has a level 36 Wailmer and level 38 Azumarill.
South of Grace, you can visit the Treasure Hunter's House, although you can't do anything there yet.
South of the Treasure Hunter's House, you can battle Swimmer Chad, who has a level 39 Golduck.
From here, you can optionally go south to Route 126. In fact, you can reach almost every location of Hoenn now, so feel free to go explore for a while.
When you're ready, go east from Route 124 to reach Mossdeep City.