The sudden inhalation of water can cause coughing, uncontrolled breathing, laryngospasm (an involuntary spasm of the muscles that control the windpipe, or larynx), flooding of the lungs with water, or the development of cardiac arrhythmia — any of which can render a snorkeler unconscious and result in drowning.

The primary risks when snorkeling are environmental hazards, equipment misuse, and physical overexertion. Strong currents and tides can pull even experienced swimmers far from their boat or shore, leading to exhaustion. Secondary drowning is a rare but serious risk where small amounts of water inhaled into the lungs can cause inflammation hours after leaving the water. Equipment issues, such as a leaky mask or a "full-face" mask that doesn't properly vent CO2, can cause panic or dizziness. Marine life encounters, such as jellyfish stings or coral cuts, can cause painful injuries or infections. Finally, many snorkelers suffer from severe sunburn on their backs and legs because the cooling effect of the water masks the intensity of the UV rays. To minimize these risks, it is essential to always snorkel with a buddy, use a "snorkel vest" for buoyancy, and wear a rash guard to protect against both the sun and stinging organisms in the water column.