The extremely high rate of tourist snorkeling deaths in Hawaii may not actually be caused by breathing in water as widely assumed, but by the lungs failing to deliver oxygen to the rest of the body (called ROPE - Rapid Onset Pulmonary Edema) leading to loss of consciousness. A combination of factors are suspected, including high breathing resistance in snorkeling masks, existing heart conditions, and recent long air travel, the three year Snorkel Safety Study concludes. In a separate study, the best indicator that the newly popular (but sometimes banned) full-face snorkeling mask was harder to breathe through was its ability to be traced to a source manufacturer
From: Metafilter