Call a body of water placid if it has a smooth surface and no waves. Call a person placid if they don't tend to make waves by causing a fuss.
Coming from the Latin placidus "pleasing or gentle," placid is most commonly used to describe a person who is not easily irritated or a body of water such as a lake that does not have waves to disturb the surface. Synonyms of placid in both meanings include calm, serene and tranquil. In other uses, placid describes something with little disruption — like "a placid neighborhood."