I had a water birth with my son. The water was quite clean. The same way we don't bleed into our bath water during the monthly cycle - there was no bleeding into the water. My midwife had assured me that should I happen to poop, she would scoop it all out. LOL!
They added salts of some sort before I went into the birthing tub to keep the water clean.
The temperature never dropped but for the life of me, I can't tell you how they kept it warm - I had bigger problems: LABOUR PAINS!!
I'm planning for a water birth again - it's all I know. I remember while I waited for my midwife to arrive, I tried various things to relieve the pain and nothing worked quite as well as when I hopped into the shower. I couldn't bear being out of the shower - I stayed in there for a few hours until my midwife arrived and I was ready to go into the birthing tub.
From that experience, I do know that water is an effective method for pain relief. I didn't tear and my baby came out clean - with only that waxy substance. I saw no blood.