The giant turtle battle sub, one of the most notorious weapons of the Second War, is really not a vehicle at all. It is a harness and cockpit that fits atop a live giant turtle (with an attached mental control rig). From within the pressurized compartment, the pilot issues commands to the creature as though it were a mount. The battle sub also includes a short-ranged cannon or rocket launcher capable of damaging ships, but only if the giant turtle surfaces.
In the Second War, the Stormreaver clan captured giant turtles for this purpose and pacified them with powerful spells of control. Since that time, goblins have bred giant turtles to be calmer and more docile. These turtles, which the goblins use for their subs, are tame and do not require magical control.
Note: The giant turtle battle sub was known simpy as the "giant turtle" in Warcraft II, but that term has come to refer to the turtle species rather than the submersible.