WordNet sense 1 (not agitated; without losing self-possession):

serene, unagitated

WordNet sense 2 (serenely self-possessed and free from agitation especially in times of stress):

composed

WordNet sense 3 ((of a body of water) free from disturbance by heavy waves):

placid, quiet, smooth, still, unruffled

WordNet sense 4 ((of weather) free from storm or wind):

calm