Water will normally freeze at 32 F but I believe for the water to freeze instantly it has to be 20 F or below 20 F.
If I remember correctly a few years ago it was 10 F outside one morning and I put some water out for a stray cat and the water instantly started turning to slush and then ice.
It was turning into a slush as I was pouring the water into a bowl for the cat.
So I would say at 10 F water should start to freeze instantly but it can happen at even lower temperatures or maybe sometimes above.