The Sunnah is to hasten to break the fast as soon as the sun goes down. There are several ahaadeeth from the Prophet (peace and blessings of Allaah be upon him) concerning that, such as the following:
Al-Bukhaari (1975) and Muslim (1098) narrated from Sahl ibn Sa’d that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The people will continue to do well so long as they hasten to break the fast.”
Al-Nawawi said: In this hadeeth we are urged to hasten to break the fast, after checking that the sun has indeed set. What this means is that the ummah will continue to be in good shape and will be fine, so long as they adhere to this Sunnah. If they delay (breaking the fast) then this will be a sign of corruption that they have fallen into.
Al-Muhallab said: The reason behind that is so as to avoid making the day longer by taking from the night. It is also kinder to the fasting person and gives him more strength for worship. The scholars are agreed that this hadeeth refers to when it is confirmed that the sun has indeed set by visual sighting or by the testimony of two witnesses of good character – or of one such witness according to the more correct view. End quote.
Another reason for that:
Is so as to hasten to eat that which Allaah has permitted, for Allaah is Most Generous, and one who is generous likes to see people hasten to enjoy his generosity. So He likes His slaves to hasten to enjoy that which Allaah has permitted to them as soon as the sun sets. Al-Sharh al-Mumti’, 6/268.
This hadeeth is also a refutation of the Shi’ah who delay breaking the fast until the stars appear. This was the view of Ibn Daqeeq al-Eid.
The Sunnah is for the fasting person to break the fast with fresh dates; if he cannot find any then with dried dates; if he cannot find any then with water. If he cannot find any then he should break the fast with whatever food and drink are available.
If the fasting person cannot find anything with which to break his fast, he should break his fast in intention, i.e., he should declare that his fast is over even if that is delayed, thus he will have hastened to break the fast and followed the Sunnah in that.
Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’ (6/269):
If he cannot find water or any other drink, or any food, he should intend in his heart to break his fast, and that is sufficient.
Based on this, if this doctor cannot break his fast with fresh or dried dates, then he can break his fast with water. If he cannot do that because he is so busy doing the operation, then it is sufficient for him to have the intention of breaking the fast, and he can act in accordance with the Sunnah in this way.
And Allaah knows best.