Depends on the game
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After getting the 8th Gym Badge at Sunnyshore City, head North towards the beachy section of the City, and talk to Sabrina ( I think that's her name - lady that will be standing there). She will then give it to you. :)
Team Galactic will block the lake untill you defeat gym leader candice; she's in snowpoint city. Just go past into the city, rest your pokemon and take your gym challenge. When you won go back and lake acuity should be free to enter
First, beat Tate and Liza at the Mossdeep Gym and receive your seventh badge, allowing you to teach your Pokemon Dive. Then Surf to Route 126. Dive when you see Dive-able water and you will be in a huge underwater cavern. Go as far south in the cavern as you can and you will find a cave entrance. Go through it and you will be in a small underwater room. Use Dive again and you will be inside the crater of a dormant volcano: the city of Sootopolis. After solving the conflict between Groudon and Kyogre, go to the small island in the center of the city. The final Pokemon Gym will be there. Juan is the Leader. Here are his Pokemon: Luvdisc (Level 41) Whiscash (Level 41) Sealeo (Level 43) Crawdaunt (Level 43) Kingdra (Level 46) He's pretty strong if you have weak Pokemon, so watch out.
In Pokémon Platinum, the Gym Leaders are in Oreburgh City, Eterna City, Hearthome City, Veilstone City, Pastoria City, Canalave City, Snowpoint City and Sunyshore City however when you first get to Oreburgh City, the Gym Leader there will be in the Oreburgh Mines and when you get to Hearthome City, the Gym Leader will first be in the Contest Hall.
-piplup (for gym 1) -starly (for gym 2 and 4) -ponyta (for gym 2, 6 and 7) or houndoor (available for gym 6 and 7) ponyta (catch in top of oreburgh city grass) / houndoor (catch outside lake valor grass) -shinx (for gym 3 and 5) -gible (for gym 8) -munchlax (additional) or other