The mood of the poem is sad and dark, describing where the bones of someone's father are laying (at the bottom of the ocean.). Alliteration helps convey this mood because the repetition of sounds, especially the 'f' and 's' sounds, make a sound that resembles the rush of waves or the sound you hear if you're underwater.
The first example is in the first line, the 'f' sound is repeated in "Full Fathom Five thy Father lies". Then, for a second example, in lines 4-7, the 's' sound is repeated in "Suffer a Sea-change / Into Something rich and Strange. / Sea-nymphS". Again, those two sounds represent the sound and feeling of being underwater where the body is resting.
This is third person point of view, omniscient - you're seeing Mario from the
outside instead of hearing him say "I am reading a book."
