The speaker's similes link "Time" to the seascape. The same simile opens and closes the poem: the speaker sees "Time flowing like a hundred yachts." At first glance, this simile simply presents the grand vision of a procession of ships, each moment a bright-sailed boat flitting past. But the striking verb "flowing" connects those imagined yachts to the ocean they sail on. Time is both boat and current here, a parade of passing moments and the medium that carries those moments at once.
The important image of Time as seawater recurs when the speaker muses that Time "drives through bone and vein" in just the same way as "water bends the seaweeds in the sea." Time rushes over people and leaves them behind, in other words: the "weeds remain" where they are, and people stay planted in their own little patch of time, unable to follow very far into the future.
The speaker also imagines time "piercing" the world "like the quince-bright, bitter slats / of sun" that shoot through the waters of the harbor. The vision of Time as "quince-bright" sunlight raises images of golden ripeness (a quince is a yellow, pear-like fruit). But those "slats" (or bars) of sun are also "bitter" as a crabapple. In this image, Time can spear you through with simultaneous beauty and pain.
For rare, small moments, though, people might be able to step "out of Time" and into the present moment, fully inhabiting what's right in front of them. When the speaker has such a moment, they reflect that they feel as if they're standing in something "like the fainter land / Lensed in a bubble's ghostly camera." Inhabiting the present, in other words, is like living in the reflections on the surface of a bubble, existing in a fragile, beautiful now-ness. That simile prepares readers for what happens next: bubbles, inevitably, pop, and the speaker can only stay "out of Time" for so long before the "suck of sea" pulls them right back into the current.
The poem's final simile is the only one that moves away from the seashore. That makes sense: it's the simile the speaker uses to describe how Time reaches far, far into the future, beyond the time when the shore the speaker stands on even exists. The speaker imagines Time tiptoeing through the dark "blindly and softly, as a mistress might" in order to keep his "appointments with a million years." This image of Time as a woman creeping toward a meeting with her lover suggests that there's something furtive and stealthy about the way Time moves. Time is somehow in love with the distant future, but like a "mistress" having an affair, it's not loyal to that future. When it reaches a million years from now, it'll still be creeping toward a meeting with a million years from then.