If it is related to the heart, the explanation could be the burden to the heart from a surge of oxygen
demand due to the swimming exertion, possibly the
cold water effect, and smoking inhibits good oxygen flow. "Cold stress acting over a period of half-an-hour to several hours causes the blood to become more concentrated and increases the risk of a heart attack," said Professor Keatinge. "Shorter exposures to cold may make people more resistant to it, but there are risks.