The poem seems to describe a scarecrow dressed as a mariner, standing in a field of maize. Its body is made of rope, which the poem calls "mar-line," and stuffed with straw. The title, "The Watch," fits: this figure is a guard, but instead of watching crows, it stares out at the sea. The poem doesn't say how a sailor's figure ended up among crops, so the image feels strange and a little dreamlike. The field and the water seem to blur together.
The speaker mostly observes rather than comments, and the tone is quiet and cold. The sea is "surging," the gulls are black, the squalls "bloated," and the final line settles into blue evening. Judging by the imagery, there is a sense of loneliness and endless waiting, as if the figure can only stand and look. The repeated "b" sounds in "Blacker," "Black," "Bloated," and "Blue" give the lines a heavy, brooding beat, and the darkening colors bring the poem to a still, ominous close.