The answer is: to evoke sadness within the reader
This stanza, as well as the whole poem, evokes a state of sadness and, perhaps, resignation to what happens in life. The author emphasizes the verse "The tide rises, the tide falls" by repeating it twice, which can be a reference to life with its "ups" and downs" or the birth and death.
By using the words "darkens" and then "damp" and "brown", the speaker is trying to create a dreary and gloomy mood within the poem.