The speaker is an old sailor who has come to rest far from the water. He calls himself "captain of sand," a title that sounds both proud and a little rueful, since he now commands only dry land. His harbor lies "a mile into the country," so his place of safety has drifted away from the shore entirely.
What he feels seems to be tired relief. He hopes there will be "no port" where he is, meaning no pull toward departure, no ships calling him back. The line "I am out of rust" is odd and interesting. It could mean he has no energy left, or that he has worn through the toughness that kept him going. Either way, he is spent. The last lines say it plainly: he is "weary / of answering the sea." The sea reads almost like a voice that keeps making demands, and he no longer wants to reply.
The mood is quiet, dry and slightly wistful. There is humor in the wordplay, but underneath it sits the calm of someone choosing stillness over a lifetime of calling and answering.