"Full Fathom Five" takes its title from Shakespeare's The Tempest. (The phrase has since been used in other texts and mediums.) In Act One, Scene Two, Ariel sings that Ferdinand's father has drowned. In Plath's poem, "Full Fathom Five," the speaker (Plath) describes her absent/dead father ("old man") in terms of love, hate, and mystery (Plath's father died when she was eight). She does so using the imagery of the sea. The speaker describes the old man surfacing rarely and compares his white hair and beard to the foam on the crests of a wave. The description is similar to mythological characters (perhaps Aphrodite or Poseidon), making the old man/father emerge from the sea powerfully but he also appears to be part of it. Thus, the sea is personified as her father. The speaker compares her father's death/burial to his being submerged in the water; hence the mystery of him being gone but coming back with the tide. When he resurfaces from the sea (and/or in her mind and memory), she has trouble believing that he is really gone and buried, no longer an influence on her life: 

The muddy rumors

Of your burial move me

To half-believe: your reappearance

Proves rumors shallow

All the speaker's conceptions of her father are related to the sea. The sea is her father's "kingdom" and even on dry land, she cannot escape his influence; for good or bad, this does seem to reflect a love-hate and/or mysterious relationship with her father: 

I walk dry on your kingdom's border

Exiled to no good. 

She is in her kingdom (dry land) but can still not escape (exile) the influence of her father (represented by his kingdom of the sea). 

In "Suicide Off Egg Rock," Plath describes a man about to commit suicide. He leaves behind him the world of factories, hot dogs, and gas tanks: the "landscape of imperfections." Life in the world (on dry land in this description) has become meaningless, glittering "like blank paper." This leaves the sea as a paradoxical death and escape from the "landscape of imperfections." 

In both poems, the sea has dual, or multiple meanings. In "Full Fathom Five," the sea represents the speaker's father, one whom she views with love, hate, and mystery (obscurity). In "Suicide Off Egg Rock," the speaker views the sea, through the man's eyes, as death but also as an escape. Also, in both poems, Plath uses the land and sea to represent two different worlds. In "Full Fathom Five," her world is the land, the sea is her father's - whose influence she can not escape even while on land. In "Suicide Off Egg Rock," the land is the imperfect world of living and the sea is death, an escape, and in the most abstract the sea is an unforeseen world: at least in the attempt at suicide or in the event of death and/or afterlife, although this is not overtly indicated in the poem.