A submarine is a real-life underwater vehicle used in the Mushroom Kingdom.
In the Muda Kingdom area, Mario can use his submarine, the Marine Pop.
The course, The Mighty Cannonship's boss uses Bowser Jr. using his Koopa Clown Car for an underwater battle, adding a air tank, which gives it a submarine-style machine.
The Koopa Clown Car's air tank is used again in All Aboard!, with Bowser Jr. as the boss again in New Super Luigi U.
Submarines are seen in Mario Kart 8. They appear in the course Water Park, where they are controlled by Nokis. Also a stamp can be unlocked in the game.