(of a body of water) free from disturbance by heavy waves "a ribbon of sand between the angry sea and the placid bay" - quiet, still, tranquil, smooth, unruffled
Not easily irritated "not everyone shared his placid temperament"; "remained placid despite the repeated delays" - equable, even-tempered, good-tempered
See also: calm, good-natured, placidity, placidly, placidness