placid in WordNet English dictionary

1. not easily irritated
   "an equable temper"; "not everyone shared his placid temperament"; "remained placid despite the repeated delays"
   equable  even-tempered  good-tempered  placid 
   
2. (of a body of water) free from disturbance by heavy waves
   "a ribbon of sand between the angry sea and the placid bay"; "the quiet waters of a lagoon"; "a lake of tranquil blue water reflecting a tranquil blue sky"; "a smooth channel crossing"; "scarcely a ripple on the still water"; "unruffled water"
   placid  quiet  still  tranquil  smooth  unruffled