Icefall Cave

Once you're in Icefall Cave, surf across the water and go deeper. In this room, you'll find a puzzle with many blue ice tiles. They are just thin pieces of ice that crack the first time you step on it, and break the second time. This is a fairly easy puzzle. You want to get to the bottom-right corner of this room, where you'll find HM07 - Waterfall. You want to fall through certain holes to get to the HM, and after some trial and error, you'll get to it.

After getting the HM, teach it to a Pokemon, exit this room and go up the waterfall. Go up the ladder, into the entrance, and you'll find Lorelei defending the Pokemon of this cave from Team Rocket. She'll then drag you into the argument, and you'll have to fight a Rocket member. After the battle, the Rocket member tells you that they've kept the stolen Pokemon in the Rocket Warehouse on Five Island. The Rocket Warehouse requires two passwords, and you only have one from eavesdropping on two previous Rockets. Instead of going to Five Island now, you'll need to first go to Six Island to find out the other password.

Locations (Icefall Cave)
Delibird Golbat Gyarados Lapras Seel Shellder Sneasel Staryu Swinub Zubat