Example : 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.

Synonyms : quiet, smooth, still, tranquil, unruffled

Example : An equable temper.
Not everyone shared his placid temperament.
Remained placid despite the repeated delays.

Synonyms : equable, even-tempered, good-tempered