A boat, a beach and a tide that does not wait: Thomas Moore builds a whole philosophy of loss from a scene you could sketch in four lines. In the first stanza the speaker simply watches. In the morning a "bark" (an old poetic word for a boat) moves "gloriously on" across the water. Returning at sunset, the speaker finds the boat "still there" but "the waters were gone." Nothing dramatic has happened. The sea has only done what it always does, and that is the point.
The second stanza explains the picture. The boat is our own early happiness, and the vanished water is whatever carried it along. Moore's phrase "the spring-tide of joy" works on two levels. Spring is the season of youth and beginnings, while a spring tide is a high, powerful tide, which ebbs as surely as it rises. The verbs are gentle but final: waves we "danced on" at morning "ebb from us" and leave us "at eve, on the bleak shore alone." Notice that the poem does not say the boat sank or was wrecked. It is stranded, which may be harder to bear. The boat survives, but it can no longer do what boats are for. Moore seems to suggest that growing older is less about destruction than about being left high and dry.
The grammar quietly draws the reader into this. Stanza one is all "I", a single observer. By stanza two the pronouns are "our", "we" and "us", and a private sight has become a shared fate. Stanza three shifts again, to a rhetorical question ("who would not welcome that moment's returning") and to "his" soul, a general figure for anyone who has felt passion arrive. The poem moves from a single sighting to everyone's experience.
The final image is surprising because the sea disappears. Instead we get fire: a soul "like the wood that grows precious in burning," giving out "all its sweets to love's exquisite flame." Moore does not name the wood, but the idea is of timber that releases its sweetness only when it burns. That image complicates the longing in the question. If the sweetness comes from the burning, it cannot be had twice, since wood that has given out its sweets is ash. The wish to have that moment back is therefore both natural and impossible. Water and fire both describe the same experience, one as something that carries us and then withdraws, the other as something that consumes us as it gives. The poem ends warmly, on a bright word, "flame," after the "bleak shore", so the memory of passion finishes up almost as consolation, even as the poem hints it cannot be recovered.
Sound reinforces the sense. The lines have four beats and a rolling, lilting rhythm, with two unstressed syllables between most of the stresses, like a wave's regular swell: "I SAW from the BEACH, when the MORNing was SHINing." The rhymes run ABAB, and Moore alternates feminine endings (shining/declining, returning/burning) with single-syllable masculine ones (on/gone, known/alone, frame/flame). The longer, softer endings have a trailing, falling quality, and the short ones land like a door closing. The most important of these is "gone," which ends the first stanza and sits at the pivot of the whole poem. Likewise "alone" closes the second stanza on the loneliest word available. The musical regularity suggests this was meant to be sung, though that is an inference from the form, not something the text states.
What makes the poem worth reading is its economy. Moore does not announce that youth passes, a point every reader already knows. He lets us see a boat on dry sand and feel the truth before it is explained. The image is so simple that it stays in the mind, and the shift to fire in the last stanza shows that feeling lost is also feeling that was fully lived.