A submarine (潜水艦) is a vehicle used to travel underwater. They are mainly used by the Red Ribbon Army.
During the General Blue Saga, when Bulma finds out that she does not have a Submarine capsule on her, Goku goes off to find the ball underwater by himself leaving Bulma on an island. Seeing that the Dragon Ball was too deep underwater for Goku to swim down and retrieve it, they went to Kame House to see Master Roshi and got one in exchange for Bulma's Micro Band. Roshi's submarine was wrecked during the journey, which resulted in Bulma giving Roshi a diamond as compensation for the destruction of his submarine.
General Blue and his soldiers use submarines to chase the Dragon Ball Gang to the Pirate Cave. General Blue's submarines also appears in the movie Dragon Ball: The Path to Power. To escape the collapsing Pirate Cave, Bulma, Krillin and Goku used a submarine found in the cave.
In a filler on Namek, Bulma uses a Capsule Corporation submarine to retrieve a Namekian Dragon Ball in the water, although because of it being among a nest of crab eggs, this nearly got herself killed by the eggs aggressively protective parent.
In the Alternate Timeline seen in Dragon Ball Z: The History of Trunks, Master Roshi, Turtle, Oolong, and Puar are hiding in a submarine near Kame House. Roshi planned to have his submarine surface so he could fight the Androids at Pepper City, but he was persuaded not to go by Oolong, Puar, and Turtle.
Video game appearances
Roshi's submarine is seen inside the Pirate Cave in Dragon Ball: Advanced Adventure and Dragon Ball Z: Attack of the Saiyans. Goku and his friends use Roshi's submarine to go in the Pirate Cave Dragon Ball: Origins 2. Pirate Submarines are seen Dragon Ball: Revenge of King Piccolo.