Because that the direction of water ... /face heading is like one of those magic water findin' sticks.
Here's the valid commands for /face
/face Turns you to face and look at your selected target
/face nolook Faces your target without changing your vertical view angle
/face fast Immediately turns your character to face and look at your target
/face fast predict Immediately turns your character to face and look at your target's estimated position
/face fast nolook Immediately turns your character to face your target without changing your vertical view angle
/face item Faces and looks at the item from /itemtarget
/face fast item Immediately faces and looks at the item from /itemtarget
/face loc y,x,z Faces the given loc (note: no spaces in 'y,x,z' portion)