Hydrophobia describes a condition where the body's tissues actively expel water to the point of dehydration, the patient becomes anxious or panicky when provided with liquids to drink, has difficulty swallowing, and has an unquenchable thirst. It is a classic symptom rabies and is responsible for the "foaming at the mouth" that appears in the late stage of the disease.
In the episode Histories, the condition is incorrectly described as a "fear of water", which is the literal meaning of the Greek translation. A fear of water is more properly called Aquaphobia.