This page explains how to find Waterfall (TM99) - a Water-type HM (Hidden Move) that allows you to climb up waterfalls in Pokemon Brilliant Diamond and Shining Pearl (BDSP).
Waterfall - Move Details
|Water||Physical||80||100||15||The user charges at the target and may make it flinch. It’s also one of the Pokétch’s hidden moves.||TM99|
Waterfall HM Location
You can receive HM Waterfall after obtaining the Beacon Badge from Volkner, the final Gym Leader. Once you’ve got all 8 badges, head to the beach near the north end of Sunyshore City, where Jasmine and the entrance to Route 223 are. Speak to Jasmine to obtain the Hidden Move and corresponding TMs from her.
How to Use Waterfall
HM Waterfall can be used while surfing and you’re facing towards a big waterfall. Simply press the A button next to the waterfall to use the Hidden Move. Or bring up the Hidden Moves app on your Pokétch and select “Waterfall”.
That’s for ascending waterfalls; to descend a waterfall, you don’t need to do anything extra. Just swim into the waterfall to automatically travel down.