The "Walther boat" used in the game is really a type XXI, that somehow had the Walter turbine. But the XXI boats without the H2O2 turbines could acheive 17 knts submerged, which is pretty good. It was propelled by 2 SSW 2500 bhp electric motors when submerged that coupled with it's streamlined hull made these speeds available.
It was the Wa201 type XVII that was deemed the true "Walter boat" in reference to Dr. Helmuth Walter. The H202 was broken down using a catalyst to provide steam and oxygen, them mixed with water and diesel fuel to produce combustion. The system worked superbly and was not a time bomb at all. One was even used by the Royal navy after the war and it was renamed the HMS Meteorite.
