Full Fathom Five Poem by Sylvia Plath

This poem is a little bit difficult to interpret. I believe she is using fathom in terms of the nautical measurement, since I can ascertain the rest of the piece is describing the qualities of the ocean. I am quite fond of the last verse 'Your shelled bed I remember, Father, this thick air is murderous. I would breathe water' It is almost like the ocean, being a metaphor for her Father, with the sandy bottom being the last thing she remembers, and with water being the air, suffocates her.