From Sonic Retro
The Sea Fox (Sea Tail 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 pops out of item boxes in Tidal Plant Zone for Tails (whereas Sonic, on hitting the same boxes, gets his Scuba Boots). 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.
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.