Water is one of those ingredients that is essential to any cocktail, but is typically given very little thought. The source of most water in cocktails is melting [ingredient=ice ice] or [ingredient=soda-water soda water]. 

While I'd love to come up with some witty spiel about water and cocktails, I think the folks at Cooking Issues have done it here. The point is that shaking a cocktail with ice is more than just chilling it. If you were making a cocktail and put all the ingredients in the freezer and got it down to proper drinking temperature, it would taste off compared to those same ingredients shaken with ice. Water's dilution of alcoholic strength makes cocktails more palatable than drinking their component spirits straight. 

Some popular cocktails containing Water