The nature of a sonnet is that the problem raised in the first part (the first quatrain in a Spenserian sonnet) is resolved in the last part (the third quatrain in a Spenserian sonnet) and strongly restated in the ending couplet. Therefore, to find the "inner meaning" of a Spenserian sonnet, look to the third quatrain and the final couplet. A quatrain is four related lines and a couplet is two related lines; a sonnet is fourteen lines in total.
Spenserian sonnets have been confirmed as the poet, Edmund Spenser, chronicling the difficult courtship of his lady love, Elizabeth Boyle. So it is correct to say that in the first quatrain, the poet (instead of "the speaker" who may not represent the poet) lays out a problem (the tide washing away his words), while in the second quatrain, his lady love introduces a reversal in logic by contradicting his concern. The third quatrain has another reversal of logic with Spenser correcting her assertion and establishing the solution to the problem presented in the first quatrain. This solution is then elegantly and strongly stated anew in the ending couplet.
From this we can say that the inner meaning of the poem is that when all the others in the world are dead and gone ("subdued") their "love shall live." Why? As the third quatrain explains, it will live because he will immortalize ("eternize") her "virtues rare" and instead of writing her name in sand by the tidal flow, he will write her "glorious name" "in the heavens."
Spenser constructed such intricately laced stories that give logical reversal (e.g., "Sonnet 75") or, at other times, logical accord because he "links" his rhyme together with three couplets that introduce both a rhyme change and idea change. Spenser's sonnet rhyme scheme is ababbcbccdcd ee. The couplets occur at the 4th and 5th lines (bb) and the 8th and 9th lines (cc) and at the 13th and 14th lines (ee). At these couplets, Spenser introduces new ideas that are closely related and either logically oppose each other for logic that reverses on itself or logically support each other for logic that evolves through the quatrains to the final solution in the ending couplet.
[For more, see Edmund Spenser, Amoretti and "Epithalamion", Arnie Sanders, Goucher College.]