Antifreeze contains chemicals that protect your engine against rust and corrosion, as well as freeze-up and boil over protection.
Water is required to activate the chemicals in the inhibitor package.
Furthermore, adding water to antifreeze actually increases the freeze-up and boil over protection provided. This would limit the corrosion and freeze up protection and heat transfer capabilities of the antifreeze. Yes, water from your garden hose will work just fine. Some European car manufacturers recommend that phosphate free antifreeze be used in their vehicles. The reason is that the water in Europe has an extremely high mineral content.