Depending on the contaminants, it's more likely a few degrees below 0C but you point still stands. Fish are removed as contaminant but minerals and pollution likely is not.

And normal water takes quite a bit of heat extraction to actually freeze if at 0C, maybe the device does not even extract enough. But you want to be on the safe side of course since clogging up your heat exchanger with ice (which expands) is not great.

(edit: and as noted in other reply pressure is a thing)

Moving water will get much colder than 0℃ before turning solid. -10℃ or even -25℃ are easily possible. If the water is also under pressure, it can get even colder.

The opposite effect is also why thermal stations (including but not only nuclear) are usually on the coast or near large rivers: you can dump a lot of water heat into water and have it carried away.

Not always good for the local ecosystem without mitigation, but at least one Japanese reactor allowed local colonisation by tropical fish and local legend said the same about Sizewell.

Sizewell C claims to plan recover waste heat and use it for carbon capture somehow, about which all I can say is a big old hmmmmm.

It is, since the obvious alternative to taking the heat from water would be taking the heat from the air or from the ground.

The air is colder in winter than the water, and the ground only provides a limited amount of heat before you can't extract any more. So water beats both.

It is a bit relevant because if the air was warm enough you would be better building huge air source heat pumps.

And if it was really warm enough you wouldn't need heating in the first place.