Is there any specific reason of why my dog often vomits after drinking water?
Both of my dogs do the same thing, and most of the time it's just a burp that surfaces before the water reaches their stomachs. If your dog is vomitting just water, then that's the problem. If his food is coming up as well, then it could be an allergy to whatever you're feeding him.

I don't mean to scare anyone but if this seems to really bother your dog, it may be suffering from early symptoms of the Megaesophagus. In some breeds this condition is hereditary, and it's more common in larger dogs. The Megaesophagus is difficult to diagnose and has limited treatment options. Usually, dogs suffering from Megaesophagus will also have fever, foul breath, and troubles swallowing large amounds of food. If you think vomitting happens too often or there are other worrying signs, I suggest that you take your dog to a vet. If you document all the symptoms, it should help the vet.

Read more about the Megaesophagus here: http://en.wikipedia.org/wiki/Megaesophagus