**Core Concept**
Hydrocution is a rare but potentially fatal condition that occurs when a person is exposed to cold water, leading to a sudden cardiac arrest. This condition is often seen in individuals who are intoxicated or have a pre-existing heart condition.

**Why the Correct Answer is Right**
Hydrocution occurs due to the sudden exposure of the body to cold water, which causes a rapid decrease in the body's core temperature. This leads to a vagally-mediated bradycardia, which can progress to cardiac arrest. The mechanism is related to the stimulation of the vagus nerve by the cold water, which in turn slows down the heart rate. The cold water also causes peripheral vasoconstriction, which increases the afterload on the heart, further contributing to cardiac arrest.

**Why Each Wrong Option is Incorrect**
**Option A:** Electrocution in water is known as "electric shock drowning" and is a different entity from hydrocution. It occurs when an electric current passes through the body while in contact with water.
**Option B:** Dry drowning is a condition where a person dies from aspiration of water into the lungs, despite not being submerged in water. It is not related to cold water exposure.
**Option D:** Postmortem immersion refers to the process of a body being submerged in water after death, and is not related to the sudden exposure of a living person to cold water.

**Clinical Pearl / High-Yield Fact**
It is essential to note that hydrocution can occur even in shallow water, and the risk is increased in individuals who are intoxicated or have a pre-existing heart condition. This emphasizes the importance of being cautious when handling water, especially in situations where individuals may be at risk.

**✓ Correct Answer: C. Drowning in cold water**