Waterbending-powered submarine

The Water Tribe Submarines are naval vehicles designed by Sokka of the Southern Water Tribe. They first appeared in The Day of Black Sun, Part 1: The Invasion.

Overview
After Ba Sing Se fell to the Fire Nation, Sokka made plans for the invasion of the Fire Nation on the long-awaited Day of Black Sun, when a solar eclipse would disable the Firebending abilities of Firebenders. It was hoped that this attack would allow Avatar Aang to defeat the Fire Lord and end the War. However, they needed vehicles to get the invaders to the Fire Nation. Sokka personally designed submarines (Although he drew them quite badly!) and presented his plans to the Mechanist. Despite the difficulty he had interpreting Sokka's plans, he figured them out and manufactured the Subs for usage by the invasion force.

Design
The Water Tribe Subs are powered by Waterbending, and appear to be fairly large craft. They each can hold dozens of warriors and at least one Earth Kingdom Tank. They have fairly good armor and are gold-plated. They have a fish-like design and are totally waterproof. They can easily propel through the water, dive, and surface with the help of Waterbenders. They are also armed with Ice Torpedoes, a missile-like device that are ideal for sinking ships or blasting through barriers. (nick.com)

Weaknesses
The only major weakness in the Subs are their lack of oxygen. The Subs can only stay under water for several minutes at any given time before they must resurface to replenish the oxygen tanks. Unfortunately, this happens on a frequent basis.

History
On the Day of Black Sun, these Subs were deployed by Water Tribe boats after they were seized by Firebenders. The Subs blasted through the Great Gates of Azulon and then proceeded towards the Fire Nation Capital. The Subs carried the entire invasion force towards the city while Aang flew there alone to face the Fire Lord. Once they were on shore, the Subs released the Tanks and the invaders, who stormed the Royal Plaza. The Subs were defended by Tho, Due, and Huu, Waterbenders of the Foggy Swamp Tribe. They stayed behind on the shores while the invaders proceeded onward.

After the eclipse ended, the Fire Nation drove back the invaders with their air force and then went towards the shore to destroy the Subs, thereby trapping the invaders. Huu, Tho, and Due did their best to destroy the bombs dropped by the airships, but they were overwhelmed and all of the Subs were destroyed. Aang and his friends managed to escape, but everybody else was captured.