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 and used to transport Earth Kingdom and Water Tribe soldiers and benders into the harbor of the Fire Nation Capital.

History
They were exclusively seen in the Invasion of the Fire Nation. Otherwise, they aren't seen for the rest of the series. 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 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.

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 on Appa, but everybody else was captured. Despite their destruction, it is very likely that these submarines will pave the way for future underwater warfare in the world of Avatar, or quite possibly serve as a kind of transport.

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 Earthbending Powered 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. At the front bottom-half of the submarines are the armored doors. Troops and tanks would exit and enter from here while the sub is on dry land.

Weaknesses
The only major weakness in the Subs is their inability to store oxygen. The Subs can only stay under water for an hour or so at any given time before they must resurface to prevent the passengers from suffocating. This is sufficient when planned for, but limits their underwater range. This weakness, aside from their torpedoes and the like, is shared by real-world humanity diesel-electric submarines, especially World War 2 submarines.