[RogerL]

Hard water will boil at a higher temperature, but it's only a degree or two.

[bromuro]

Plus, pasta start cooking at 80 ºC. No need to keep the water boiling like crazy :)

[RandallBrown]

Boiling like crazy or not, it's still just 100C if it's boiling at all isn't it?

I guess at a really low boil there could be cooler parts of the water.