young strong guys in their late teens, early twenties. As they emerge from anesthesia, they can have a laryngospasm where their vocal cords slam shut, with how strong their diaphragms are, they can end up creating too much negative pressure in the chest cavity and causing pulmonary edema. Usually self limiting, and with some diuretics, and supplemental oxygen it will resolve pretty quickly.

Key here is these people who dies snorkeling are not healthy at all. The person in this article is definitely obese, likely has bad sleep apnea, possibly diabetic and hypertension. Maybe he even has some coronary artery disease. Laying prone (face down) in the water, breathing through a small apparatus could potentially set him up for some increased negative pressure through increased resistance in the snorkel piece, and possibly pulmonary edema. For most of us, even mild pulmonary edema would not be an issue. Someone with his health history for sure would outstrip his reserves and be a problem.

Reality is, the real message here should be, "if you are very unhealthy, obese with poor eating habits, poor exercise tolerance, sleep apnea, you may want to think about shedding 30lbs before snorkeling".