It's most likely not the water used but the tannin in the tea.
According to wiki-
" The astringency from the tannins is what causes the dry and puckery feeling in the mouth following the consumption of unripened fruit or red wine."...and....
"The tea plant (Camellia sinensis) is an example of a plant said to have a naturally high tannin content. When any type of tea leaf is steeped in hot water it brews a "tart" (astringent) flavor that is characteristic of tannins. This is due to the catechins and other flavonoids, which are categorized as tannins by doctors, biologists, and chemists."