This Spenserian Sonnet is very similar to Shakespeare's "Sonnet 118," ("Shall I compare thee to a summer's day") in that both are meta-poetry (poetry about poetry). Both poems' speakers believe, as was common in the pre-Romantic era, that art--as much as love and beauty--makes us immortal. Spenser's speaker says, "My verse your vertues rare shall eternize, / And in the heavens wryte your glorious name." Shakespeare ends his sonnet in similar fashion: "..and this (the poem) gives life to thee..."
Sonnet 75 is set up in the typical problem-solution format, with the volta (or turn) in line 9. In the octet, the waves speak that they can wash away mortality:
Vayne man," sayd she, "that doest in vaine assay.
A mortall thing so to immortalize,
For I my selve shall lyke to this decay,
and eek my name bee wyped out lykewize.
Line 9 (the volta) begins the sestet in which the speaker offers his (and Spenser's, Shakespeare's) counter-argument:
Not so," quod I, "let baser things devize,
To dy in dust, but you shall live by fame:
My verse your vertues rare shall eternize,
And in the heavens wryte your glorious name.
Where whenas death shall all the world subdew,
Our love shall live, and later life renew.
The irony, of course, is that the speaker immortalizes the thing (the waves) that originally tries to "wash away" his footprint in the sand.