I was wrong.
Here was the rough accounting for significant components:
I admit I'm guessing a bit when it comes to the quantities here. With a bit of internet research I was able to find out how much cocoa powder and sugar you need to make chocolate milk. And I know there's more soy in this than milk.
112 gallons for the soy milk @ 28 gallons per ounce.
18 gallons for skim milk @ 6 gallons per ounce.
11 gallons for sugar @ 0.5 gallons per gram.
40 gallons for cocoa powder @ 4.13 gallons per gram.
~19 gallons for vanilla @ 33.42 gallons per gram of vanilla bean.
200 gallons total water consumed to produce a 250 calorie, 8 ounce serving of nutrition drink.
If I'd wanted something that gave me 9 grams of protein, 40 grams of carbohydrates, and 6 grams of fat, my best bet would be a bit more than an ounce of chicken, a 6-7 ounce baked potato, and a pat of butter (5 g). We could even add in a 1/2 cup of steamed broccoli (~3 ounces) and an apple (3.3 ounces) for dessert.
20 gallons for the chicken at 16.6 gallons per ounce.
13 gallons for the potato at 2.1 gallons per ounce.
6 gallons for the pat of butter at 1.14 gallons per gram.
6 gallons for the broccoli at 2.1 gallons per ounce.
20 gallons for a small apple at 6.2 gallons per ounce.
65 gallons total for a nice little meal. Maybe that's what I'll have for dinner...