How do you get HM Waterfall in Pokemon Red?

Answer 1

Waterfall is HM07 in Pokemon Gold, Silver and Crystal only. There are 2 Pokemon in Red and Blue who can learn it naturally (Goldeen and Seaking) however because its an 'old' attack, you may trade over some red or blue Pokemon, teach it to them in Gold or silver and trade them back. This is the only NEW HM that can be taught to old Red and Blue Pokemon, HM06 Whirlpool doesn't exist in red and blue.
Waterfull is not an HM in Pokemon Red or Blue. The only Pokemon that know Waterfall in those games are Goldeen and Seaking, which Goldeen can learn Waterfall at level 37, while Seaking is at level 39 in Generation I.