From Sonic Retro
The Sea Fox, known as the Sea Tail (シーテイル) in Japanese, is a one-person multi-purpose submarine used (and presumably designed and built) by Miles "Tails" Prower. First appearing in Sonic Triple Trouble's Tidal Plant Zone, the Sea Fox is also a major component of Tails Adventures, and shades of the craft's design can be seen in many of Tails' later vehicular inventions.
Sonic Triple Trouble
Blue and orange with a drill bit mounted on the front, the Sea Fox in Triple Trouble can be found throughout Tidal Plant Zone when playing as Tails, waiting for him to hop into the cockpit (whereas Sonic instead gets his Propeller Shoes from monitors). The submarine's hull is curved in this iteration, and it has a built-in torpedo launcher on the front, which makes dealing with the boss significantly easier. When the player reaches the surface, the player can jump out of the Sea Fox with or .
The Sea Fox, however, is a fragile machine. If it takes a single hit, it is destroyed, forcing Tails to wade through water the old-fashioned way. In the boss fight, it can take a few more hits before it's totally destroyed.
In Tails Adventures, the Sea Fox transforms into (or out of, depending on your point of view) Tails' Remote Robot in a technological marvel of folding miniturisation. Somewhat more angular than the Triple Trouble version, the Sea Fox can now be equipped with a variety of upgrades scavenged from the invading Battle Kukku Army, including mines, extra armour, and a rocket booster that enables the sub to take to the skies. Tails even has his own submarine lab to outfit the vehicle during this game; attached to his house on the south shore of Cocoa Island.
In other media
In the Sonic the Hedgehog comic series and spinoffs by Archie Comics, Rotor Walrus built the Sea Fox for Tails to play in, but took out the oil from it under Princess Sally's orders as she thought Tails was too young to use such a thing, though Tails made it operational with the help of an oil-soaked seagull. As he matured as a member of the Freedom Fighters, the Sea Fox would become Tails' official marine vehicle. He would only use it for a few stories in the series' earlier years before it was eventually destroyed by Octobot.
The Sea Fox's design in the Archie comics is based on its appearance in Sonic Triple Trouble.
In the Sonic Boom series, the Sea Fox is not a rideable vehicle, but is instead a mini-RC vehicle used by Tails to explore deep underwater trenches that no one else can reach in the video games Shattered Crystal and Fire & Ice. In gameplay, the player can fire missiles to clear obstacles and use a sonar to check the Sea Fox's surroundings on the Nintendo 3DS touch screen. However, the Sea Fox can only be used for a limited amount of time, and will be forced to resurface if it runs out of energy. Levels where the Sea Fox can be used are located within the adventure levels in the first game, and are their own independent levels in the second game.