Beware of swimming-induced pulmonary edema, known by the acronym (SIPE). It's preventable and treatable but can be fatal if you ignore the symptoms. "...First reported in 1989, its incidence is likely underreported and has been estimated between 1.1 percent and 1.8 percent."
[One's] age, long swims, cold water, being female, and high blood pressure or heart disease are risk factors. Doctors stress that SIPE often happens to otherwise healthy people, making it hard for swimmers to breathe and depriving them of oxygen. The precise cause of SIPE is not fully understood, but it happens when fluid builds up in the lungs without the person taking in water. The person usually experiences severe shortness of breath, low oxygen levels, and a wet cough. The risk of SIPE while swimming in open water isn't well known either. [Still, it] is likely to be uncommon, report cardiologists from the Royal United Hospitals Bath and the University of Bath, UK, after treating a woman with the condition. For mor info here is the [LINK]