His mood was calm and detached, like still water without ripples.
Tips
history
澹 is largely literary today; in modern writing the calm sense is normally carried by 淡 (bland / faint) and 恬 (placid). It also has a rarer reading tán used only in the place name 澹台 (a compound surname).
register
Literary register — appears in classical prose, the phrase 澹泊 (indifferent to fame and gain), and etymology notes, not in everyday speech.