Why did they draw water and spill it?
Rashi citing Targum Yonasan, Midrash Shmuel 13:1: They spilled their hearts like water, in Teshuvah in front of Hashem.
Rashi: This showed humility - we are in front of You like this water, which is spilled!
Radak: Perhaps this was a Siman for pardon of sins - "k'Mayim Avru Sizkor" (Iyov 11:16).
Malbim: This refers to spilling tears.
Why did they fast?
Malbim: It is to check for sins of individuals and the Rabim, and separate from them. "Zeh Tzom Evcharehu Pate'ach Charbutzos Resha" (Yeshayah 58:6).
What did Shmuel judge?
Rashi: He judged monetary cases between people, or sins that were done.
Radak: He punished everyone according to his sins, for there they confessed their sins.
Malbim: He judged sins Bein Adam l'Chavero. For this, Viduy (va'Yomeru Sham Chatanu) and Teshuvah alone do not suffice.
