All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A fasting person is ordered not to inhale water strongly lest the water should enter the throat. Laqit Ibn Sabrah reported that the Prophet said: "Exaggerate when rinsing your nose unless you are fasting". (At-Tirmithi, Ibn Maajah and others). If a fasting person takes precautions not to let water enter his throat, but doubts whether some water entered his throat, the fast is still correct and such doubts do not have any effect on the validity of fast.
Even if one is sure that water has indeed entered his throat accidentally it does not affect the fast, according to the most correct opinion of the scholars.
This rule is confirmed by Imaam Ibn Qudaamah in his book al-Mughni.
As for other questions please read the Fatawa:
83126 and 81869. Allah knows best.