A sailor stands in a field of corn, staring at the sea. The poem never says "scarecrow", but everything points there. The Mariner is "knit with mar-line and stuffed with straw", so he is a made thing, stitched from ship's cord and filled with farm leftovers. The central idea, as I read it, is a watch kept by something that cannot leave its post. He is built from the sea's equipment, stranded among crops, and facing the water he seems to belong to. When evening comes his duty does not end. The world around him just gets darker.
The title carries a lot of this. A "watch" is a sailor's shift, a guard's vigil and a lookout's job, and the word comes back as "Watching" in the third line. It also means a timepiece, which suits a poem that ends on the arrival of evening.
The construction is lean. The five lines taper from eleven syllables to eight, seven, five and five, so the poem shrinks as the light fades. The grammar thins out in step. The first three lines form one sentence with a single verb, "stands". After the colon the verbs vanish and we get fragments: "Black gulls. Bloated squalls. / Blue with eventide." The colon works like a handover, giving us what the Mariner sees, and the shortened phrases suggest the stillness of a figure who can only look. Nothing in the poem happens. Things are simply there, in the order a fixed gaze would take them in.
There is no regular rhyme or metre, and the end words (maize, straw, sea, squalls, eventide) don't chime. Sound holds the poem together instead. "Mariner", "maize" and "mar-line" open with the same hum. The hyphen in "mar-line" also lets "mar", a verb meaning to spoil or damage, sit briefly on its own. That may be coincidence, but it fits the poem's slightly worn, weathered mood. The hissing s sounds of "stands", "straw", "surge", "sea" and "squalls" suggest surf. The b sounds of "Blacker", "Black", "Bloated" and "Blue" land like bell strokes in the last two lines. "Straw" and "squalls" share a broad vowel, which is as close as the poem comes to rhyme.
The opening word is odd in a useful way. "Blacker" is a comparative with nothing to compare itself to. Blacker than what? Perhaps than the dusk, than the gulls, or than he was by day. The poem leaves the question open. The inverted order, "Blacker stands the Mariner", has an old, ballad-like gravity, and it puts the colour before the figure, as a silhouette does.
The colours then shift. The gulls are black, though gulls are usually white, so the dimming light seems to be taking the colour out of things. There is also a small irony. A scarecrow is there to frighten birds off the crop, but these birds come from the sea he faces, and a figure of rope and straw can do nothing about them. "Bloated squalls" is the most inventive phrase. Squalls are sudden gusts, but "bloated" makes them swollen and waterlogged, almost like a body. The word also echoes "stuffed with straw", since scarecrow and storm are both packed full, one with straw and one with wind.
The final line changes the palette. After two blacks comes "Blue with eventide", a colour that is lighter but also sadder, since blue is the colour of evening and of low spirits. "Eventide" is an old, soft word, and it slows the line to something like an exhale. It is also the only line that describes the whole sky rather than a single thing in it.
The poem is significant for how much it draws from so little. In five lines it builds a character, a setting, a mood and a sense of time passing. It also stays open. The Mariner could stand for homesickness, for an old sailor's memory carried inland, or for any loyalty that outlasts its purpose. The capital M turns him from an object into a type, and some readers may hear an echo of Coleridge's Ancient Mariner, another figure bound to the sea's story, though the poem doesn't insist on it. Nothing is explained, and the scarecrow keeps watching as the light goes.