Those of us who paddle the oceans rather than inland rivers have a lower chance of encountering water that is sub 50F, unless we live north or south of a certain line on the map. Those who do paddle in those icy conditions have a notoriously bad survival rate in accidents. Going as far north as Vancouver means paddling on sub 50F water and the survival rates there, compared to say the 52-55F water I commonly paddle, are low enough to be horrifying.
Oceans don't allow for "short" swims to safety. Paddling safely on the ocean often involves moving sufficiently off the coast to avoid the worst conditions (boomers, clapotis, and breaking surf). If you add ice to the mix, especially moving ice which can be between you and a shore that is notoriously devoid of shelter and resources (as you go further north), and the need for thermal protection well exceeds what one may choose for "short" swims on a river.
And even on rivers, terrain is a factor. Odds are that one will capsize in rather fast moving water, often found between high walls of a cut canyon (since fast moving water and rock often creates those kinds of conditions and these conditions prevail on many rivers here in the west). There may be beaches accessible, oxbows, warmer tributaries, etc. along the river, there just many not be one at the point of capsize.
So, my general rule is to be ready to swim for an extended period of time in the water. Strainers, foot entrapment, etc. can mean a very long time in the water before a rescue can be executed, even if the conditions do not offer an immediate threat of death.
I am not going to offer a statement that the "gasp" reflex happens or not, but I've seen people unexpectedly fall into cold water and it surely looks like the first thing they do is pop their heads above water, swim ineffectively, and try to gasp for air with their noses to the sky while panting like Lassie. Whether or not this happens underwater or not isn't really important. Anyone in this situation, moving water or not, is in a life threatening situation and they know it. Panic may well set in, decisions become instinctive (and often wrong), and the chance of survival decreases.
I just finished reading about the divers who discovered a U-Boat off the coast of New Jersey and the sad stories of those (quite accomplished divers) who died investigating or trying to take trophies from the wreck. It is a classic example of the same type of situation where, if everyone stays calm, many of the accidents will not occur, but in cases of extreme fear, everything falls apart and people die.
In one example, a father/son buddy team both died because a temporary entrapment left one of them without air and the other seriously depleted. There were rescue bottles of air available, but in the silt and fear, they ignored their compasses and swam 180 degrees in the wrong direction along the length of the hull.
Frightened people tend to make horrendous decisions and often take others with them down the path of risk. We lost a father/son duo on the coast this year because they threw a stick for their dog into a tidal rip. Neither was dressed for such conditions (52F water).
When the dog had difficultly coming back in against the rip, the son went to help the dog. The path of failure compounded when the dad jumps in to help the son. Bad decision one spawned bad decision two, both died. The dog swam out of the rip and made it to shore. Their fear, inexperience, lack of basic water safety, etc. made this an accident just waiting to happen, and I see this level of ability and judgement EVERY time I go to a beach in California.
In my opinion, we need to tailor these discussions along lines of "what if I encounter..." not along lines of, "I can survive immersion in absolute zero vaccum conditions and anyone who can't is an idiot," since those of us who can survive expected conditions probably won't have the kinds of problems that are described here.
And nature has a HUGE intolerance for arrogance. You and I may well have all the skills, judgement, knowledge, and tools/equipment needed to survive expected conditions only to find that nature decided to change the conditions on us without asking our permission.
I didn't mean to make this a diatribe, but I do want to remind everyone that life is what happens when you make other plans.