The title promises a message in a bottle, then takes the bottles away. What remains is water doing the carrying. In these four short lines, water brings "letters from the land" up into the air, and it arrives with "this fleet of ice." The word "fleet" turns floating ice into a line of ships, each one loaded with something sent from the ground.
The poem never says what the letters contain, so the reader has to decide. One natural reading is that the ice, as it melts or evaporates, holds traces of the land it passed over, and the message is the landscape itself rather than any written note. The speaker doesn't announce a big feeling. Instead the tone is quiet, observant, and a little wondering, as if they are watching something delicate and strange unfold. The mood is hushed and cool, with a faint sense of loss or drifting, since nothing here is addressed to a particular person and no one is promised a reply.