A thesis describes the main idea of the whole essay ;)
This passage uses repetition to create a haunting effect in two different ways. The first way repetition is used is through literal repetition, repeating the phrase "my darling" and ending both of the final lines with the word "sea". The second form of repetition that creates this effect is the repetition of an idea. A sepulchre is an area where a person is buried, and so is a tomb, so the final two lines have the same meaning.
The change in end rhyme in from the first two lines to the last two lines is also significant, because it changes the focus of the poem from mourning the person who has been laid to rest, to the place in which she has been laid to rest.
