The poet is using repetition to create a haunting effect. The first time repetition is used is through repeating the phrase "my darling" and ending both of the lines with the word "sea". A different way to use repetition also done by the repetition of an idea. A sepulcher is an area where a corpse is buried, and so is a tomb, so the final two lines have the same meaning.
The change in end rhyme from the first two lines to the last two lines is also different. It changes the focus of the poem from mourning a laid corpse to the place in which she will be laid to rest.