Hidden Machine 08, known as Dive, can be found at Mossdeep City. Mossdeep City is located on the northeastern area of the game's map, putting it in a location just before the HM needs to be used to reach the next vital location and thus the next badge in the game. Dive is a water-type move used to dive under the water in certain locations on the map.
HM08 is a gift given to the player following the correct sequence of story events, received from the character called Steven Stone. This character can only be found in Mossdeep City and is encountered as a part of the natural progression of the game's plot and exploration.
Mossdeep City is also home to a rare space center in the Pokémon games, and is the location of a rare Pokémon that can normally only be found there, if not traded for. Mossdeep City is a vital location to reach because of its Pokémon Gym, so it is hard to miss the Dive HM when coming to the city.
Dive is a move that attacks an enemy in battle, and is used in the overworld to reach the underwater trenches found in certain spots, indicated by dark patches in the sea. These patches are filled with special Pokémon and items that give the player a reason to explore the depths.