Dive

Dive (ダイビング Diving) is a damage-dealing -type move introduced in Generation III. Players can receive the HM containing this move in the Generation III games from Steven Stone in Mossdeep City. This move cannot be used outside of battle without the Mossdeep City Gym Badge. Dive comes back in Generation V. You recive it in Undella Town from a woman by Cynthia's Villa. You can only use it to explore Abyssal Ruins in Undella Bay.

In Battle
A two-turn attack. The user dives underwater on the first turn, then hits on the next turn. The enemy still can attack you if the enemy uses Surf, it will double the attack.

Outside of Battle
Dive allows players to go underwater in specific areas of water while surfing. This was first introduced in the Hoenn region, but was also brought back in the Unova region.

Trivia

 * Dive is sort of a -type version of the moves Dig and Fly.