On route 18, you need surf, this is also the location of one of the seven sages after you beat N.
HM Waterfall can be found on Route 18 in Pokemon Black and White.
to bad
route 18
On route 18
on route 18
yes
you get it in route 18
Somewhere on route 18
Route 18.
it is at route 18, in corner on farthest island
See related link below for a tutorial.
it is found on the ground at route 18