Stone Monkey Gene
The flowing water was the most gentle and soft energy in this world. It was continuous and endless, capable of eroding all objects. Once it stored up enough strength, nothing would be able to block it.
As for fire, it was one of the more explosive energies in the world, capable of incinerating all objects, turning them into ashes. It was also able to emit light and warmth, nourishing the myriad of creatures.
Water and fire were in mutual opposition. It could be said that they were the two sides of Taichi, capable of engendering yet also restraining each other. It could be said that these two forms of energy were the most common of all energies in the universe.
So it turned out that being impervious to fire and water didn't only mean that one would be able to resist the damage caused by the flow of water or the burns from flames. It was about being able to completely isolate the user from the properties of different forms of energy.