My mother died approximately twenty years ago, by
falling into a well that was not completely filled with water. She was a
righteous woman. Is she among those who are counted as shuhadaa’
(martyrs)?

Praise be to Allaah. 

Yes,
those who drown are counted as martyrs, because of the report from Abu
Hurayrah (may Allaah be pleased with him) according to which the Prophet

(peace and blessings of Allaah be upon him) said: “The shuhadaa’
are five: the one who dies of plague, the one who dies of a stomach
disease, the one who drowns, the one who is crushed by a falling wall, and
the one who is killed for the sake of Allaah.” This was narrated by
Maalik in al-Muwatta’, and by al-Bukhaari, Muslim and
al-Tirmidhi.

And
Allaah is the Source of strength. May Allaah bless our Prophet Muhammad
and his family and companions, and grant them peace.