Instructions
Enter the Ice Path off Route 44 near Mahogany Town
Walk (you will actually slide on the slippery surface) to the steps and head down them to the first subfloor.
Slide to the left and solve the puzzle maze by moving up, left, up, left, down, left, up and right.
Reach the man who will give you the Waterfall ability from this point by traveling right, up, left, down, left, up and then right. Take the steps back up to the first floor and speak to the character here. He will grant you HM07, the Waterfall ability.