The poem consists almost entirely of an extended metaphor about hope and disappointment—about what the future does and doesn't hold for us. This metaphor begins with "Something is always approaching" in line 3 and concludes with the end of the poem 21 lines later.
In this metaphor, the future events we "eager[ly]" await are like ships sailing toward us as we watch from a high "bluff." They "Sparkl[e]" with "promise[]" and possibility, even if they seem to approach much too "slow[ly]" (because we're impatient for their arrival). As they "draw near," they look lavish, alluring, and even sexy (as in the cheeky detail about the "figurehead with golden tits / Arching our way"). Indeed, wealth and sexual bliss might be among the "promises" the future seems to hold.
But these anticipated events seem to promise more than just money, sex, etc.: they seem to promise happiness and an end to our state of impatient desire. We believe each one will "unload / All good into our lives," as in the idiom about one's ship coming in. Instead, yet each event leaves us nursing "disappointment," like a ship that's brought us nothing more than a handful of limp weeds ("wretched stalks"). It's not that these events fail to materialize: "nothing balks," or blocks, their arrival. It's just that each event is temporary ("it's / No sooner present than it turns to past") and far less life-changing than we'd imagined.
The end of the metaphor zeroes in on the one event that will permanently change our lives: death. The poem imagines death as a "black- / Sailed unfamiliar" ship that's sailing straight for us, "towing [...] / A huge and birdless silence." In other words, whether we like it or not, death comes for all of us, and it brings nothing more than a void. The metaphor—and the poem as a whole—discourages any hope in an afterlife, just as it shoots down our grandest hopes for this life.