Why does my dog cough after drinking?

I have noticed this before as well! http://Dogster.com says "Naturally, like any of us, sometimes our dogs drink too fast or they are temporarily distracted, and a few drops go down the wrong way." Also, "If a dog consistently coughs after drinking water, there may be more to it than rapid or reckless swallowing can explain away. At least three medical issues might be the source of the problem, all of them involving the trachea:"

Kennel cough Hypoplastic trachea Collapsed trachea

Like most things, I would always take a visit to the vet to be sure it's nothing serious.

I hope this helps you!