If it wasn't your intention to purposely drink water after Sehri, then inshAllah you should be fine. Action is by Intention in Islam, so as long as it wasn't your intention to break your fast, then your fast should still be valid. Just keep more careful next time and keep fasting until it's time to break it.
Abu Hurairah reported that the Prophet said: “Whoever forgets he is fasting, and eats or drinks is to complete his fast, as it was Allah who fed him and gave him something to drink.” (Related by the group.)
Commenting on it, At-Tirmidhi says: “Most of the scholars act according to this Hadith. It is the opinion of Sufyan Ath-Thauri, Ash-Shafi’i, Ahmad, and Ishaq.”
Abu Hanifah reported that the Prophet, peace and blessings be upon him, said: “Whoever breaks his fast during Ramadan due to forgetfulness is not to make up the day later or to perform any expiation.” (Related by Ad-Daraqutni, Al-Baihaqi, and Al-Hakim, who says that it is Authentic according to Muslim's criterion.)
Ibn `Abbas reported that the Prophet, peace and blessings be upon him, said: “Allah will not hold anyone of this nation responsible for what is done in error, forgetfulness or under coercion.” (Related by Ibn Majah, At-Tabarani, and Al-Hakim.)