after you get all 8 gym badges, go to sunyshore cities beach at the way end of sunyshore and talk to the girl there, she will give you waterfall.
Chat with our AI personalities
In Shunishoart City.
Jasmine will give HM Waterfall to you at the beach after defeating Volkner.
its in the icefall cave in island4
u have to get waterfall from a girl on sunnyshore beach.
one of your party Pokemon has to know waterfall press A on the waterfall it will say it's a waterfall do you want to use waterfall say yes and you will go up the waterfall