This is my kind of topic. I have been obsessed with water and heating water, and kettles, these are very important in gong fu cha, I mean it is easy to learn how to pour water to the teapot and how to fill the cups from the teapot, but it is a lot harder to learn and experiment with water quality and heating water.
Sofar I have been experimenting with these two kettles, I know that my neighbour has an antique silver kettle, I have been ashamed to ask them for permission to use it.
Another element in water quality is the way you heat it, it is better to heat it quickly on high natural heat like charcoal fire, I use my stove top with a metal base so it does not damage my tetsubin or Lin kettle, another thing is to watch it and observe the steam exiting the spout of the tea kettle, when it is gently rising remove from the heat source and start preheating your teaware, if you let it reach roaring boil, the steam will rise like from a locomotive, you have overboiled your water, this is bad.
How much water are you using? I use the whole amount needed for the tea session, I fill for green oolong around 800 ml of water to my kettle, another method is the chao zhou gong fu cha method, where you use a small 250 ml of tea kettle, that is only twice as large as your teapot, you heat your water and use it to fill the teapot only once and you refill the kettle between each brew, this is meant to keep the water fresh, I have not tried this method due to the lack of equipment, I would need a nilu or a japanese ryoro, to heat the water that fast with charcoal.
As of water quality, we are blessed here by a dozen of great quality water sources, I have tried most of them, but the problem is that they all come in plastic containers, and it is not the same when you go directly to a natural spring and collect fresh water from the mountains, I will try to experiment with this also.
The final thought about water would be the container, in which I rest my water, the easiest is to find a huge glass jar (around 10 l) cover it in a cool dark place, the other is a japanese mizushashi that holds water in the tea ceremony, they come in either ceramic or porcelain, and finally yixing or other clay jars, Lin`s ceramic studio sells water containershttp://www.aurlia.com.tw/detail.php?id=143&lang=en
It would be a lot to order it online.