You get waterfall at sunshore city keep going straight in sunshore city and there is a girl named jessica and she gives it to you by the water
Where Do You Get Hm 7 Waterfall In Pokemon Platinum?
You get it from jasmine the girl on the sand in the top right corner of sunyshore city after beating volkner their gym leader
Talk to Jasmine, In SunyShore, She'll give you it automatically.
Sunnyshore City
You get it from Jasmine in Sunnyshore city, Right before the ocean, You'll automatically get it.