I haven't played the 3rd generation games in a while but if I remember correctly,there is no Gym Leader in Lilycove.
Chat with our AI personalities
There is no gym in Lilycove. There is a gym in Mossdeep city though. If you surf to the right, after a while you'll reach it.
In Pokémon Ruby, Sapphire, and Emerald, the gym in Lilycove City is located in the northeastern part of the city, near the beach. To access the gym, you'll need to navigate through a series of maze-like platforms that shift when you step on them. The gym leader is Tate and Liza, who specialize in Psychic-type Pokémon. Be prepared for a challenging double battle against these formidable trainers.
After beating the gym leader, head right of fortree city until you reach Lilycove City. You will meet up with May at Lilycove mart. Go back where the Safari zone is and right in front of it, you will see an opening of water. Use surf to get to Mt.Pyre. Explore it. You will receive your next objective from there.
From lilycove city if you go east you can reach mossdeep city.
Head to lilycove city then surf east to mossdeep city go into the gym and defeat the leaders for badge #7. Finally one more to go, go into steven's house which is in mossdeep city talk to him to get the HM Dive use it to reach sootopolis city this is where the last gym is!