The water — Either that which ran from the well of Harod, mentioned Judges 7:1, or some other brook.
Related Commentaries of Judges 7:4
Judges 7:4
4 And the LORD said unto Gideon, The people are yet too many; bring them down unto the water, and I will try them for thee there: and it shall be, that of whom I say unto thee, This shall go with thee, the same shall go with thee; and of whomsoever I say unto thee, This shall not go with thee, the same shall not go.