You go on to defeat the eight gym leader.
Candice
Defeat the 8th
after you've defeated the 1st gym leader
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
The 7th leader is Blaine. He uses fire pokemon.
The seventh gym leader is Candice, the Ice Type leader. She is at Snowpoint City.
Do you mean a gym leader?
You get waterfall once you've Defeated the Gym leader in SunshoreCity, After you have go to the northern beach and a girl will give it to you. Hope this helps.
Get the Rainbow pass and go to the seventh island talk to the lady at the pokemon center in the top hat, and she will lead you to looker.
The water Pokemon gym leader is in Pastoria City.
The eighth gym leader in Pokemon platinum is the one in sunny shore city.
You use your strong(est) Pokemon to kill the Gym Leader's Pokemon.