WordWeb Online

1. (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
    
2. 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