Coolant also has lubrication properties... water does not.
If you used straight tap water, I'd drain it as soon as you can, as regular tap water contains TONS of minerals which can deteriorate the internal passages of your engine block, head, water pump, radiator, heater core, etc. If you are going to use any type of water, use only distilled water in your coolant system.
You can also run a 70% (water) & 30% (coolant) mix if you want your temps to stay a little lower. This will still be a very safe mixture for areas of the country where possible freezing occurs during the winter months.