The title promises a job listing, but what follows reads more like a quiet portrait of someone who watches and waits. The first stanza sketches a worker: a night messenger with dirty boots and aches that suit the role. The tiredness isn't a complaint so much as a qualification, proof of honest labor. The second stanza widens out to a "surveyor of the sea," measuring something we want more than "the king's own spring." That phrase suggests a longing for something wilder and freer than wealth or royal comfort, though the poem never says exactly what it is.
The last stanza reads like the actual requirements, and it's the most surprising. To do this job you must be a fly, a mind, and present. A fly is small, unnoticed, and sees from the wall. A mind thinks and understands. Being present ties them together. Judging by these lines, the speaker seems to see the poet's role as humble attention. The mood is spare, wry, and gently devoted.