Talking about technical details, heres the spec for the engines....
the Sea Duck’s engines contained a special turbocharger device so top-secret, that even Baloo hesitated to use it except in extreme circumstances. Called the "overdrive", this custom-built module was capable of accelerating the Duck's engines to many times normal speed. Details are sketchy, but the Sea Duck may have been able to approach the sound barrier with the overdrive activated. However, use of the device caused intense heat to build up in the engines, the reason why Baloo only used the overdrive sparingly. Today, the overdrive no longer exists, having burned out when Baloo raced to Cape Suzette to save Kit (see "Plunder & Lightning," Part 4).
THE OVERDRIVE SYSTEM . . Built by Baloo and Wildcat, the overdrive module for the Sea Duck was one of the more astounding feats of engine technology. While specific technical details are unknown (and, it seems, undocumented), some general information is known. Incorporated into the main cylinder section of the engine is a modified turbocharger, connected to the fuel system via a special line. A button inside the cockpit activates a separate ignition system within the overdrive mechanism. Propellant accelerators increase the fuel flow to the engines, while air intake manifolds emerge from the top of the engine cowling, taking in extra air to cool the rapidly-heating inner assembly.
Overdrives are difficult to manufacture, and cannot simply be mass-produced -- each individual aircraft requires its own unique, hand-crafted drive unit. Plus, the complexity of the overdrive limits proper maintenance to only highly-skilled mechanics and technicians.