You don't need to pre-boil the water, but you should use distilled water so you don't wind up with mineral deposits building up on the heating element. (It's not just this sterilizer that would have this issue; it's common for appliances that heat water to recommend you use distilled water.) It's cheap and comes in gallon jugs. As you mention... you can also easily push the button to repeat the cycle if you worry it hasn't run long enough (the 24hr cycle uses enough water to let you run it extra if you want.)