you get it in the ice cave
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You can find Waterfall in the Dark Cave.
HM07 Waterfall is found on the Ice Path.
Get the HM Waterfall in the ice path then go to victory road click A on the waterfall and your pokemon will go up the waterfall.
After you beat the eighth gym you are allowed to use the HM waterfall
Feraligatr cannot learn Waterfall in Pokémon Gold and Silver, additionally it cannot be taught it via the HM.