Sudden immersion in cold water causes an involuntary gasp and rapid breathing. This is cold water shock, and it is one of the main dangers for anyone who falls into the sea, a river or a lake.
Float first
Water safety organizations advise that if you fall in, you should lean back, spread your arms and legs, and float until your breathing settles before trying to swim or call for help.
Don't go in after someone
If you see someone in difficulty, call 112 or 999 and ask for the Coast Guard. Throw something that floats if you can do so safely. Entering the water yourself puts two lives at risk.
General safety information only. In an emergency in Ireland, call 112 or 999.