You don't see the shot string when you shoot a shotgun pattern into the water. When you shoot at a duck on the water the first pellets that hit way below the duck are the bottom pellets in the pattern. The pellets that hit in the water way above and way past the duck are the top pellets in the pattern. A pattern with no shot string that is completely flat will hit the water the same way a long shot string would, bottom pellets low and below the duck and top pellets above and past the duck.
Below is a low quality drawing side view of a 24" flat pattern with no shot string hitting the water. The pellets you see hitting the water 30 feet to 60 feet apart is not the shot string.
What you actually see is the spread, magnified by the shallow angle of the shot as the pellets enter the water. The bottom pellets in the pattern enter the water first and the shot may travel another 15 or 20 yards until the uppermost pellets hit the water. This delayed entry gives the illusion that the shot column is being graphically displayed when in fact it is merely a projection of the pattern diameter.
