The title "Medium" does double duty here. A medium is someone who speaks with the dead, and it's also a substance something travels through, like water. The poem seems to use both meanings. The speaker goes to a grave "to speak with him," as if conversation with the dead were possible, but what follows suggests it doesn't go as hoped.
Instead of an answer, we get a picture of a boat in a flood. The image feels precarious and adrift, as though the speaker is being carried along by grief rather than steering through it. Nothing is said about what the dead man replied, which may be the point: the silence is the response.
The last line is plain and human: "an ache for my own bed." After the strain of reaching toward the dead, the speaker just wants comfort, rest, and the ordinary safety of home. The mood is quiet, tired, and sad, but also honest. Grief here is not grand. It is exhausting, and the poem admits that.