A cup of tea is more than 99% water. So good water is as important for a good brew as the tea itself. Fine teas are delicate. A contaminant like chlorine, high concentration of minerals (i.e. hard water), or a poorly washed teapot can affect your enjoyment of good tea.
Ideally good water means fresh spring water with a neutral pH level (i.e. close to 7) and have just enough minerals to give it a round taste. Most experts believe mineral concentrations ranging from 60 - 180mg of total dissolved solids per litre of water and magnesium to calcium ratio of 1 to 3 with under 10mg of magnesium per litre of water is ideal. That said, we have tried soft water with Magnesium to Calcium ratio exceeding the recommended ratio and found that to be good enough for our teas. Many bottled spring water brands in Asia fulfil these criteria.
If your tap water is too 'hard' (i.e. high concentration of minerals), you may want to use bottled spring water, following our guidelines above. If your tap water has an acceptable level of minerals, use a good activated carbon water filter to remove unwanted chemicals such as chlorine, fluoride or other chemicals that may affect the taste of your tea. We found most filtered tap water in most Asian cities acceptable, but not so for many European and American cities.
On the other end of the spectrum is distilled water. Although distilled water, with their lack of minerals can bring out more flavours in the tea, they impart a raw, sharp edge to the tea, making it taste 'rough'. We want some minerals to 'round up' the taste of the tea. If suitable mineral water is not readily available and your tap water is too hard even with a water filter, you may mix suitable proportions of filtered water to distilled water to achieve the desired mineral concentration. Its often trial and error with water.
Finally, do not over-boil the water. Boiling water removes the dissolved oxygen in water. Dissolved oxygen makes the tea taste more 'lively'. When we brew tea, we typically add just enough water to the pot so reboiling the same water is minimised.