tranquil in WordNet English dictionary

1. not agitated; without losing self-possession
   "spoke in a calm voice"; "remained calm throughout the uproar"; "he remained serene in the midst of turbulence"; "a serene expression on her face"; "she became more tranquil"; "tranquil life in the country"
   calm  unagitated  serene  tranquil 
   
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