There are quite a few examples in this poem, but I can point out two. Firstly, an alliteration is the use of multiple words that have the same starting sound in them, usually starting with the same letters.
In line one, when the author writes, "half so happy in heaven", the "h" sounds in the words are stressed, creating an alliteration.
Similarly, the last line says, "sounding sea", which stresses the "s" sounds to create alliteration.