The speaker looks back at a spot where two people used to meet, out on the sand, in a place that the sea has now covered. The title, "Solastalgia," names a particular kind of homesickness: the grief of watching a place you love change or vanish while you are still living there. The poem seems to hold that feeling, and it ties it closely to the end of a relationship.
The second stanza is the strangest. The place is where "that busy and sorrowful / answer to questions / was most clear," which could mean the restless sound and motion of the water, as if the sea itself replied to whatever the two of them were asking. Then comes the break-up, and the last lines place it at the same spot. The word "sunk" carries both meanings: the ground is underwater, and the love has gone down with it.
The mood is quiet, wistful, and heavy. There is no anger here, only the ache of a loss that happened twice, to the place and to the people.