If you have salts building up around the faucets in your house (as I do), don't use tap water. The salts will precipitate out and coat various places of your cooling passages that you really don't want caked with salt. Distilled water costs less than $1/gallon at Kroger. If you use it, you won't be wondering if your tap water is okay to use.
Also, when removing the drain plug from the block, keep a firm grip on it. The pressure of the water coming out is a bit more than what you get from draining oil, and it will push the plug out of your fingers and lodge it some place around the engine mount that will take hours to find.
I use BMW coolant because it's not that much more expensive than any other coolant, and the local BMW dealer is five miles from my house. Plus, the Bentley manual warns against using coolants containing phosphates. There's a LOT of debate about coolant on this site. Use whatever you want. Just don't leave it in the car for 10 years.