6' down is a tough shot for even a veteran shooter.
I seldom shoot that deep but I'd say I hit 1 or 2 for 10 when they are that far down. The best way to learn that is to take a 2 liter bottle and fill it full of sand. Tie a rope around the neck with a buoy on the other end so you can retrieve it. Put it in about 3' of water and shoot at it. If you miss, tighten your line but leave the arrow stuck in the bottom. Look down the string and notice how much it bends when it hits the water. That is from the diffraction of light when it hits the waters surface. Fish are actually closer to you than they appear. The geneal rule is about 4" for every foot down, but in varies. Now back to the 2 liter...odds are you missed a foot or more high. Wind your arrow up and aim lower and repeat untill you can hit the bottle 50% of the time. Them move to 6' of water and repeat. For this deep you will need to aim close to 2' low if not more. Practice makes perfect.