as calm as a millpond (of sea) = λάδι, γαλήνια, ήρεμη
The sea is as calm as a millpond today. = Η θάλασσα είναι λάδι σήμερα.
Τhe sea was a brilliant blue millpond (Oxford Dictionaries)
Antonyms:
choppy, rough, full of waves, turbulent, heavy, heaving, storm-tossed, stormy, tempestuous...