(as) calm as a millpond

Extremely calm and untroubled, as of water. Though a millpond is specifically a pond that supplies water to power a mill wheel, the term "millpond" can be used to describe any area of calm water. It's so peaceful to just stare out at the water when it's as calm as a millpond like this. Thank goodness the ocean was calm as a millpond for our boat ride today—I get seasick really easily! I'm OK with the kids playing out there today because the lake's been as calm as a millpond.