In all of my hort classes, I was always taught to remove water sprouts, as they are generally "useless."

I did some digging, and couldn't find much directly related to pruning water sprouts.

Here's an abstract that was published in HortScience in 1998:

Effect of Upright Shoot Removal on Fruit Size and Maturity of Four-year-old `Braeburn' Apple Trees
P.I. Garriz, G.M. Colavita, H.L. Alvarez and A.J. Alvarez
Facultad de Ciencias Agrarias, Comahue National Univ., c.c. 85, 8303 Cinco Saltos, R.N., Argentina

The effect of upright shoot removal on the progress of maturity and fruit size around the normal picking time were evaluated for apple fruits cv. `Braeburn' at the Experimental Farm of the Comahue National Univ., Rio Negro, Argentina, during the 1996Â97 growing season. The experiment was conducted on 4-year-old trees, spaced 4.0 x 2.3 m and trained to palmette leader. Average fruit load was 80 fruits per tree. Five trees per treatment were randomly selected; they were: 1) select cuts: water sprouts were eliminated entirely at 78 days after full bloom (DAFB) and additional removal of newly formed shoots arising from the scaffolds was performed at 120 DAFB; 2) control: trees received no cuts. From 158 to 184 DAFB, a sample of four fruits was taken from each tree at weekly intervals. Fruit size and internal quality were measured. Loss of leaf surface by shoot removal significantly decreased fruit mass by 4.74%, for all data combined (P So it appears there are some negative effects.

I also found a dissertation from Cornell which addresses summer pruning, but it didn't specifically study water sprouts. From scanning it briefly, it appears that heavy to vigorous summer pruning can have negative effects on the trees.

Here is a link that might be useful: Dissertation

Presumably, judging from the managment system, the trees in the experiment were full dwarfs which would mean that the ratio of leaves to fruit would naturally be small. My guess is that the affects of removing water sprouts might be far more dramatic than on trees with more vigorous root-stock.

Cornell does recommend the removal of water sprouts in the center of the tree from mid-July to mid-August in NY. This helps reduce fungus pressure and may steer some extra calcium to the fruit.

You should be clear that we are discussing sprouts connected to big wood and not the "pencils" forming near the fruit off smaller wood. These pencils definately feed the fruit.

I summer-prune hundreds of mostly very old and large apple trees every year (I actually spend almost 7 months of the year pruning fruit trees full time) without obvious ill affects. On some of these trees I do major structural pruning and if it affects the brix the results are subtle.