The poem is a few lines about arriving somewhere strange and refusing to be fazed. A castaway lands on a new shore, and the first thing the speaker notices is how people behave there: "every man goes / as if at dinner." Even in an unfamiliar place, people move with the calm of someone heading to a meal, as though nothing could be more ordinary.
The ending is the odd, striking part. Having been "cast up" by the water, the speaker says, "I believe no sea / besides the ship's." That could mean the speaker trusts only the sea they knew from the ship, or that the wild ocean that wrecked them feels unreal now that they are on land. Judging by the wording, the speaker is shaken and a little numb, holding onto the one version of the sea they understand.
The mood is quiet, dazed and slightly eerie. Nothing dramatic is described, yet the calm surface makes the disorientation feel stronger.