What is the ruling on breaking the fast with only water, and does it invalidate the fast?

Intentionally consuming water or other anullers of the fast breaks one's fast. However, if done forgetfully, it does not break the fast — according to the most authoritative opinion — because the Prophet, peace and blessings be upon him, said: "Whoever forgets while he is fasting and eats or drinks, let him complete his fast, for indeed, it was Allah who fed him and gave him drink."