Alliteration helps convey the mood of the poem because it is describing the bones of someone's father resting at the bottom of the ocean. 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.
Two examples are in the first line, the 'f' sound is repeated in "Full Fathom Five thy Father lies". 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.
