I think we've discussed this before.  It basically has to look / feel / act like water according to common human perception.  Now, the more "polluted" it is and the less it resembles water, the less likely it is to constitute valid matter.  Thus, if the water is slightly mirky, most likely still valid.  If it's thick mud, less likely, etc.  But things like chemicals and pollutants that are largely imperceptible to the senses would not invalidate the Sacrament.  Tap water, despite the presence of chroine, chemicals, and other toxins, etc. ... would still constitute valid matter for the Sacrament of Baptism.  If your ordinary person would look at it, touch it, taste it, and say, "yep, that's water," then it's water.

By "hot or cold" is meant that the temperature doesn't matter and is not of the essence for validity.  Its temperature doesn't change the essence of water ... though I imagine that its state would, i.e. if it were frozen into ice or boiled into steam, but even then I'm sure touching ice to the skin would cause a tiny amount of it to melt into water, or a tiny bit of steam condensing on the head would not be difficult to achieve.  It has to flow on the skin and act like water, which obviously solid ice or steam can't readily do.

I don't think the canonists were trying to create a binary, "either the water has to be cold, or it has to be be hot, but it can't be just warm".  That would create a type of sorites situation, "how hot is hot?" and "how cold is cold?".

Yeah, it just means "temperature of the water doesn't matter".