Yep, there sure is... but most likely not enough.
Water gets into a cocktail by way of melting ice. Ice, and the water that comes from it, are key ingredients in cocktails. Without said ingredients a cocktail for the most part would be unpalatable.
One issue I have noticed is the lack of shaking or stirring that goes into cocktail mixing. One must shake a cocktail for a good 10 seconds and stir a cocktail for a good 20 seconds to get the proper amount of melted ice (water) into a cocktail.
There is a current trend of very sweet, high fruit juice cocktails. I think that this trend has come about because of the lack of water that has found its way into drinks as of late. If a drink is properly shaken or stirred less mixers would be required to make the drink balanced enough to drink.
So the recipes of many modern drinks (invented in the past 25 years) may actually seem watered down if one was to shake it the right amount. If you are a person that does not like sweet drinks you might want to try some of the new modern drinks like the Cosmopolitan, but just shake the drink more.
One can't be afraid of watering down a drink when it comes to cocktails. The water is there for a reason.
Some bartenders will say, "Hey I can't stand around shaking and stirring all day long. There is a line of customers at the bar!" Yes, it is true that the 10 to 20 seconds extra it will take will slow down the process, but it is that extra mile and the great cocktails that will bring your customers back time and time again.
In high volume bars you can pretty much expect an under watered drink. But if the bartender has time... they should do'em up right.
So how much water should be in a cocktail? It really depends on how much water was intended to be in the drink in the first place. In general as far as classic cocktails is concerned I think that after the ice is melted and poured in the glass the water content is between 1/4 and 1/3 of the drink. So in a 6 oz cocktail about 1.5 oz is water.
The point is, use the water that melts in a cocktail as an ingredient not as a way to just cool off the drink. Many classic cocktails that by today's standards may seem undrinkable were initially concocted with the water in mind. So those drinks become very nice cocktails if the right amount of water is present. In my opinion the amount of stirring or shaking should be indicated in the recipe to help alleviate this problem.
When mixing your own drinks, stir and shake your cocktails as much or little as you like. Because what is most important is that you enjoy your drink.