Where is the secret waterfall in the legend of Zelda a link to the past?

you will need to swim for this, and youll need the 1st power glove.

go to the witch's hut on the north east of the map. go through the massive white block on the right.

follow the path until you come to the entrance were you purchased the flippers. don't enter that area

stay on the shallow area ad walk left until you come to the highest point, then swim forward, swim into the water fall

and there you go, that's the secret waterfall :)

presuming you meant that one :)