“I faint when I of you do write, Knowing a better spirit”
'Faint' here means to grow weak or lose courage, not to lose consciousness. The poet is not physically dizzy: he is paralyzed by the knowledge that a superior poet ('a better spirit') is also writing in praise of the same young man. 'Spirit' in Renaissance usage denoted a person of exceptional creative power.
historical The Fair Youth, the Dark Lady, and the Rival Poet“Your shallowest help will hold me up afloat, Whilst he upon your soundless deep doth ride”
'Soundless' does not mean silent here: it means impossible to sound — too deep to measure with a lead-and-line, the Elizabethan navigator's tool for gauging depth. The rival poet sails the profound depths of the young man's worth; the speaker manages only the shallows. The contrast positions the two poets at opposite ends of the ocean of the youth's excellence.
“He of tall building, and of goodly pride”
'Tall building' means a ship of impressive height and construction, not a structure on land. In Elizabethan English 'building' could denote the make or frame of a ship, and 'tall' described a vessel with high masts and imposing hull. 'Pride' carries its older sense of splendid appearance or fine bearing, not arrogance. The line completes the contrast: the rival poet is a great man-of-war or merchant vessel; the speaker is the small bark of line 13.
“if he thrive and I be cast away, The worst was this,—my love was my decay”
'Cast away' holds two senses at once: (1) shipwrecked, a vessel driven onto the rocks and lost — the literal term within the nautical conceit running through the poem; and (2) rejected, thrown aside, abandoned by the young man in favour of the rival. 'Decay' likewise works double: ruin or destruction of the ship, and the speaker's undoing or ruin as a poet and lover. The couplet turns both senses quietly: if the poet is wrecked and ruined, the cause was his love — offered as a dignified epitaph, not a complaint.
historical The sonnet form