There are 2 times when a deer develops tremendous thirst - one is when he is running from danger, the other is when he is facing an opponent in combat. When he is hunted, he will take to the river and stay submerged as long as he can hold his breath. He will then swim downstream in the middle so as not to touch the branches on either side. Sometimes he will stay completely submerged except for his nose.
In a chase on land, he becomes so faint that he longs for the refuge & refreshment of the waters. As intently as a hunted animal longs for refuge and refreshment, that is the way we are supposed to long for the Presence and the righteousness of God!