Waterbears can go without food or water for more than a decade. They can survive temperatures from zero to above the boiling point of water, pressure six times stronger than the deepest ocean trench, radiation hundreds of times higher than the fatal dose for a human, and the vacuum of space.
They aren’t able to just survive “zero”… they’re able to survive -273 degrees Celsius. Why not any colder? Another half degree colder would be absolute zero, which would turn the water bear (and anything else) into a completely different state of matter that does stuff that only quantum particles do! So the water bear doesn’t mind the cold, but the atoms can’t handle it.