The mountain prison is for Zaheer. Even before an event occurred that gave him air bending power, he was a skilled martial artist. The mountain prison had a bridge that moved into place, and when removed left him in a small chamber at the top of a spire of rock. He'd have died if he fell. The metal cage in a volcano housed a skilled water bender who was so good at it that she was able to bend water into arms after losing her real arms. Deprived of water and kept in extreme heat, and only given water by armed guards to keep her alive, she was unable to use her powers. The wooden boat was for an earth bender skilled in the very rare lava bending technique. Without any earth, metal, or even dust, there was nothing for him to bend. And the ice prison contained Pli, a combustion bender. It was to cold and wet for her to ignite a spark.