Question

Where can I find the HM for Waterfall in Sapphire?

Answer 1

You get it before you fight Kyogre in the Cave of Origin.

Source: http://www.serebii.net/pokemon_advance/tm-hm.shtml

Comments

I thought it was given to you after you beat the last gym leader (which is after you catch Kyogre/Groudon)

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i beleive thats in emerald