The speaker is drifting downstream, carried along by the current rather than steering. Turning to look, they notice a wreath of flowers floating close by. The title, "Floatsam," puns on "flotsam," the debris that rides on water, and suggests the speaker is a kind of drifting object too.
Then comes the question, set apart from everything else: "On whom had it lain last?" A wreath usually belongs to a funeral or a grave, so the speaker is wondering about a dead person, someone now unknown. The question is quiet and unanswered, and the space around it makes it feel heavier.
The mood is calm, hushed and a little eerie. Nothing is stated outright, but the poem seems to hint that the speaker and the wreath share the same fate, both moving along without control. Judging by the imagery, the speaker may be thinking about death, and about how easily anyone's memory or mourning can be carried off and forgotten.