What is the expiation for intentionally drinking water during the daytime in Ramadan due to a panic attack, fear, and shortness of breath?

A fasting person is permitted to break their fast for a legitimate Shar’i (Islamic legal) excuse, such as an illness that makes fasting difficult, worsens it, or causes it. If the questioner broke her fast due to a legitimate Shar’i excuse, there is no sin upon her, and she is only required to make up the missed days, based on the Almighty’s saying: "And whoever is ill or on a journey, then [he should fast] a number of other days."