The world's most venomous fish is the stonefish, which is commonly found in the shallow waters of the Indo-Pacific region. The stonefish's venom is stored in the spines located on its back, and it is released when pressure is applied to them. The venom is incredibly potent and can cause severe pain, paralysis, and even death in humans. It is essential to exercise caution when in waters where stonefish may be present and to seek immediate medical attention if stung.