(ANS) Contrary to what everyone else has said here, in fact it is definitely possible (high risk) to have a heart attack
if the water temperature is very cold
**Especially on a summers day where the air temperature is high or higher than the water temperature. (The bigger the temperature differential the higher the risk).
Why? because the body can receive a shock
to its fundamental biological systems and this puts extra-ordinary stress
& strain on the heart & cardiac system. The body suddenly is forced into survival mode and works to preserve the bodies core temperature to keep vital organs working.
NOTE: People die every year in the UK from falling into cold water or lakes or rivers. I'm NOT saying you definitely will or that heart attack is a certainty it just not a good idea to jump into cold water as it shocks the body & increases the risks of some kind of sever cardiac event.
**You just die faster if the water is ice cold.