Open English Wordnet
Adjectives
(s)placid, quiet, still, tranquil, smooth, unruffled(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”
(s)calm, unagitated, serene, tranquilnot 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”