An underwater-to-underwater missile is a guided missile that is fired from one submerged submarine, travels at high speed to the area of the distant submarine, and then delivers an antisubmarine payload, usually a torpedo. Payloads also have included nuclear depth charges; the U.S. UUM-44 SUBROC carried a W55 thermonuclear warhead. The Russian SS-N-15 STARFISH/RPK-2 Viyuga could deliver either a Type 40 torpedo or a nuclear warhead.
With the early generations of these weapons, one assumption was that since flight through the air could be far faster than through the water, the hybrid weapon could reach its target much faster than a torpedo. The greater range possible with a hybrid weapon also gave more safety to the launcher of a nuclear weapon.
The Russan BA-111 Shkval (Russian for "Squall") was a radically new approach, and it is arguable if the first versions could be considered guided. Essentially, it created an underwater bubble (i.e., area of cavitation) in which it could travel under rocket power, at speeds greatly faster than a torpedo. The first versions simply had an autopilot to keep the weapon on a straight line, and, given the target, assumed to be close, had no time to maneuver, terminal guidance might not be necessary. Terminal guidance would be especially unnecessary if the weapon had a nuclear warhead.
Another special case of UUM could involve the launcher not being a submarine, but some type of moored mine that releases a torpedo or rocket-assisted torpedo.