Why does Sylvia Plath decide to behin her poem crossing the water with black and repeats it twice in the first line?
Black can sometimes be a metaphor for death or sadness. You might want to try and google the poem. Some sites have explanations or information on the poems. You could also do a biography search on Sylvia Plath, she is a well known author and I think you could benefit from doing as so.
Enjoy the world of poetry, remember that poems always have a true meaning under the flap:)