A letter arrives, and the speaker compares it to a ship loaded with salt. Salt is a good image here: it preserves things, stings wounds, and stands for tears. The letter seems to carry all of that, and the poem suggests it is full of anger. The speaker calls it "so angry a note / to deliver to the dead," which hints that the sender is writing to someone who has died, or that the letter reaches the speaker as if from, or for, the dead. The poem doesn't say which, and that uncertainty is part of its unease.
The last stanza turns bleak. The salt has "no light," and the ship is "deserted by her fleet," left alone and cut off from the others. The speaker seems to feel stranded with a grief or anger that has no answer and no company. The mood is cold, quiet, and lonely. In only a few short lines, the poem makes a letter feel like a weight drifting in from far away, impossible to unload and impossible to reply to.