The poem sets two versions of a river side by side. Below the surface there is "a sweet green night," a calm, cool, almost soothing darkness. Above it, the day is "rank," a word that suggests rot and a heavy smell. The usual order is flipped: night feels pleasant and day feels foul.
Then the speaker brings in something half-believed. "People say" there is a ship down there, "fitted to go to the dead." The speaker doesn't confirm it, so the ship remains a rumor, a story passed along rather than a fact. The title points to a burial, and the river becomes both a grave and a passage.
The mood is quiet, eerie, and oddly tender. The speaker seems to prefer the hidden green depths to the unpleasant world above, and treats death less as an ending than as a journey that has already been prepared for.