If I am wearing Tefilin, and I feel very thirsty, should I take off my Tefllin to drink, (if I am even allowed to interrupt) or am I allowed to drink water while wearing Tefilin?
The Shulchan Aruch (OC 40:8) rules that אכילת עראי temporary eating is permitted while wearing Tefillin. Drinking water would seem to fall in this category.
In Yerushalmi Ber. 22b two Beraisos are mentioned concerning the t'fillin blessing, one says once (אית תניי תני מברך םעם אחת) and the latter twice (אית תניי תני פעמיים) a day.
A large meal while wearing t'fillin (הא אכל ואינון עילוי) would be considered an interruption that diverts attention from t'fillin and thus requires the blessing to be repeated, as referred to the latter Beraisa.
According to Rabbi Z'eira Rabbi bar Yirmiyah interpreted the first Beraisa as referring to a snack (עראי) which does not constitute an interruption.
Nor does a glass of water, we may suppose...