Waterfall is a teachable HM move that allows water-type Pokemon to climb up waterfalls.
In Super Smash Bros. Brawl, Waterfall is Squirtle's Up Special Move. It appears that a torrent of water appears that Squirtle "surfs" on top of. The attack does multiple hits with minor knockback towards the end. It is difficult to interrupt as a recovery move, as the hitbox is large, disjointed, and directly infront of Squirtle. It is also possible that when used you can change it to move more vertical or horizontal by tilting left or right on the control stick,or pad.