Hypoxic blackout, also called underwater blackout syndrome, is a distinct and preventable cause of drowning. The sudden and unexpected death of a young, fit swimmer or diver, almost always male, necessitates the consideration of a differential diagnosis which includes four syndromes—preexistent cardiac disease, electrical conduction abnormalities, epilepsy, and hypoxic blackout. The pathophysiology of hypoxic blackout (overriding the carbon dioxide sensor by presubmersion hyperventilation) may be induced by autonomic conflict between cold shock diving reflexes in certain predisposed individuals. Death occurs in both public and private swimming pools and in the sea, and case series include those training for underwater hockey, synchronized swimming, free diving, and playful submersion endurance challenges. The sole preventive stratagem is advocacy for awareness of risks, suitably targeted to “at risk” groups.