Submarine

A submarine is a type of underwater transportation, used in the Mario World and on the the real world.

Super Mario Land
In the Muda Kingdom from Super Mario Land, Mario can use his submarine, the Marine Pop.

New Super Mario Bros. U
Although a real submarine doesn't appear in New Super Mario Bros. U, 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.

New Super Luigi U
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.

Mario Kart 8
Many submarines are in Mario Kart 8. They appear in the course Water Park, which they are more submarine looking, and are controlled by Nokis. Also a stamp can be unlocked in the game.

Appearance
The Marine Pop from Super Mario Land has an unknown color, as the game doesn't give out color; just black and white. In New Super Mario Bros. U & New Super Luigi U, Bowser Jr.'s Koopa Clown Car holds the same as the clown car, but has an additional air tank or hood. However, in Mario Kart 8, they look just like real submarines, running on tiny wheels and controlled by Noki's.

Trivia

 * Super Mario Land & Mario Kart 8 are the only games featuring a real submarine.