Shelley gives time no clock face and no calendar. He gives it a mouth. In ten lines, time becomes a vast sea that swallows people, grows sick on what it has eaten, and howls for more. The poem is a single address to this monster, and it ends on a question nobody can comfortably answer: "Who shall put forth on thee?" Who, that is, would dare to set sail on a life that must be lived on such water?
The opening metaphor does most of the work. "Whose waves are years" turns the passing of time into something physical, a swell that rolls under us whether or not we notice it. Then the poem darkens. The sea's waters are "of deep woe" and "brackish with the salt of human tears." Brackish water is only partly salty, a murky mixture, so the word suggests that time isn't pure grief but is steadily contaminated by it. The rhymes support this. "Years" is paired with "tears" and "woe" with "flow," so the poem's sound ties the calendar to crying and sorrow to movement.
The sea is also alive, and unpleasantly so. It "claspest the limits of mortality," an embrace that is really an enclosure, since nothing human exists outside it. It is "sick of prey, yet howling on for more," a vivid image of an appetite that can't be satisfied even when it has been overfed. Then comes the bluntest verb in the poem: it "vomitest thy wrecks" onto the shore. Time devours people and then throws up what is left of them. The archaic "-est" endings (claspest, vomitest) give the address a formal, almost prayerful tone, which makes the ugliness of the image stranger. Shelley speaks to the sea the way one might speak to a god, but accuses it like a prisoner in the dock.
There is a puzzle in the middle of the poem. The sea is "shoreless," yet two lines later there is an "inhospitable shore." The most natural reading, since "its" seems to point back to mortality, is that the shore belongs to human life, not to time. Time has no edges, and we are the ones who get washed up on land we didn't choose. The line that carries this idea, "Vomitest thy wrecks on its inhospitable shore," is also the longest, running to twelve syllables against the usual ten. It sprawls past the expected length, as a wave overruns a beach.
The form is otherwise controlled. The first four lines alternate their rhymes (years, woe, tears, flow), then the poem moves through a rhyming couplet (more/shore) and an unpaired line, "Treacherous in calm, and terrible in storm." Nothing answers "storm," and that fits the line's message: even calm is a trap, and there is no safe condition to rhyme with. The final three lines shrink to six syllables each and return to the opening cry, "Unfathomable Sea." The poem begins with this word and ends with it, so the reader finishes where they started, circling the same unmeasurable depth. "Unfathomable" is the key word. It literally means "unable to be sounded," and it implies that time can't be measured, mastered or understood, however many clocks we build.
The closing question is the poem's real argument. "Put forth" is the language of setting sail, and the question is rhetorical, with no answer given. Having said the sea is treacherous in every weather, Shelley leaves the reader with a dilemma, since nobody gets to stay on shore.
The poem is a good example of how much a Romantic poet could do with a single extended metaphor and a sharp ear. Shelley died by drowning in 1822, and it is tempting to read the poem in that light. But the poem as supplied carries no date, so nothing in it can be tied to that event, and it works without any such help. Its power comes from how directly it makes an abstract idea frightening, by turning time into a body of water, then a creature, then a threat to the reader.