This year was no exception. The evening television news on Christmas Day invariably features clips of those hardy annuals brave enough to venture for a Christmas swim.
Snug as we are beside the glowing Yuletide embers, let us conjure up vicariously the exquisite discomforts of their feat, concisely described, incidentally, by P.J. Hartigan in The Forty Foot - a Monument to Sea Bathing.
A swimmer diving into water, no matter what the time of year, is transported instantaneously to a foreign medium. In normal circumstances on dry land, a person standing erect has a vertical column of blood in the veins extending from the feet to the heart.
The weight of this blood creates hydrostatic pressure that results in a disproportionate fraction of the circulating blood accumulating in the lower limbs.
But on immersion, the entire surface of the body is exposed to a uniform external pressure that opposes this internal norm. There is an immediate shift of blood from the legs to the chest, the heart has to pump more blood at each beat, and there is a consequential cascade of adjustments to many other organs in the body.
Simultaneously, breathing becomes more difficult because of the external pressure on the chest.
There are additional complications when the water happens to be very cold. The body's response to this sudden thermal jolt is a temporary cessation of breathing, a dramatic change in heart rate and, thereafter, possible hyperventilation.
In extreme cases, a prompt decrease in blood flow to the brain may cause disorientation or a clouding of consciousness, causing the swim stroke to be ineffective.
In addition, if there is a tendency to hyperventilate, conscious control of breathing may be interrupted at a time when it is needed most, rendering it out of step with any waves that may approach the swimmer, with obvious, potentially very serious, results.
After some time in cold water, the main risk is hypothermia, a significant drop in the body's core temperature. The average person can ward off hypothermia for perhaps 30 minutes in Irish winter waters.
In doing so the swimmer must rely on constrictions of the blood vessels, which reduce the rate at which the internal heat is carried to the body's surface, on the insulation provided by layers of fat and, most important of all, on restricting the time of immersion in the chilling medium.
Surprisingly, perhaps, swimmers lose the most heat through their heads; up to 50 per cent of that generated by the vigorous exercise.
It is for this reason, presumably, that nearly all those Christmas swimmers wear their swimming caps.