A Submarine is a large, mechanical Heartless. It has glowing yellow eyes and a spherical, black head encased in a clear dome. There is a periscope on top of this dome. The Submarine's main body is comprised of two sections that resemble large jaws with sharp, triangular teeth. The upper jaw is blue and the Heartless's head rests on top of it. The lower jaw is sky blue and protrudes outward slightly. There is a small, yellow, diamond-shaped light in between the two jaws. There are two small engines with yellow, cross-shaped propellers on its body, one on either side. It has three "fins", two long, relatively thin pectoral fins on its sides, and one shorter, wide tailfin. All three of its fins are sky blue with blue tips. The Submarine's overall appearance is similar to that of an anglerfish. The Heartless insignia is on the back of its head.
The Submarine's name references the underwater vehicle, which reflects its aquatic nature.
Its method of attack is similar to the Hex Ring Nobody Ship in that it uses Ring Shots, which can easily be dodged by staying in one place. They have high defense and Endurance, making them quite hard to defeat, but when defeated, they grant a reward. These Heartless can quickly be identified by the parascope they bear above the waters, and can give time to prepare for the attack.