The theme of Sonnet 75 by Edmund Spenser is that no one lives forever, but his lady's virtues and their love for each other will be immortalized forever in the poem. The waves of the ocean in the poem represent the passing of time, and the writing in the sand represents the lady and their love.
Edmund Spenser wrote a sequence of sonnets entitled "Amoretti," of which Sonnet 75 is a part. Also titled "One Day I Wrote Her Name Upon the Strand," Edmund Spenser weaves a tale about the ocean, love and immortality. He writes about being at the beach with his beloved and writing her name in the sand. As expected, a wave comes and washes away her name. He tries it again with the same result. The woman speaks up and states that it is unreasonable for him to continue writing her name as the waves will just continue washing it away. She compares that to her mortality, that eventually she will be erased from the earth just as her name is erased from the sand. The speaker in the sonnet then replies that he will make her immortal by writing about her in his poetry. That way, their names and love for each other will live on forever.