Answer:
Even though the night is quiet, the sea is not
Step-by-step explanation:
In the poem "The Tide Rises, the Tide Falls" before the statement "Darkness settles on roofs and walls, But the sea, the sea in the darkness calls;" the writer described the sea tide that it rises and falls which implies that the sea is constantly moving at that moment which practically means the sea is making sounds, even though everywhere was quiet or silent at that time (night).
Hence the best summary of this line is that "Even though the night is quiet, the sea is not"