placid

adjectives

(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, smooth, still, tranquil, unruffled
is derived from
placidness
not easily irritated
"an equable temper"; "not everyone shared his placid temperament"; "remained placid despite the repeated delays"
equable, even-tempered, good-tempered, placid
is derived from
placidity
is derived from
placidness