Pokémon Mystery Dungeon: Blue Rescue Team and Red Rescue Team/Solar Cave

After beating Rayquaza you should have access to Solar Cave after Wishcash gives you dive.

The Dungeon
From floors 2-14 you should find a key on floor 15 there’s a chamber with either a link cable dive or waterfall in it. On floor 20 which is the last floor you could find surf or a link cable.