How do you get Dive in Pokemon Pearl?

To get Dive in Pokémon Pearl, you must first beat the sixth Gym Leader, Volkner, and receive the Rise Badge. Dive can then be found on Route 213, located on the way to Victory Road. Use Surf to reach the north side of the lake, enter the house, and talk to Cynthia's grandmother inside. She will teach you Dive, which lets you explore underwater areas.