flosi

The raft came first. Floso is a raft, and flosi is what a raft does: it sits on the water and goes where the water goes. So the verb is not a borrowing in its own right but the ordinary Esperanto move of putting -i on a noun root, and the meaning follows from the object. That also explains the shade of the word - flosi is passive, carried, at the mercy of the current. A thing that flosas is not swimming.

Three near-identical verbs, and the difference is worth pinning down. Naĝi is to swim: the swimmer supplies the effort. Flosi is to float or drift: the water supplies the movement. Flui is what the water itself does. The river fluas, the log flosas on it, and the boy naĝas beside the log. Miss the distinction and you get la rivero flosas, which says the river is bobbing along on top of something else.

Two more that sit one letter away. Flori is to bloom and flui is to flow, and neither is related to flosi despite the shared opening. In handwriting and in fast listening they are genuinely confusable, so the safest habit is to fix each one to a picture: flori a flower, flui a tap, flosi a log on a lake.

Derivations. Flosigi is to float something, to set it adrift; flosanta is floating as an adjective; flosponto is a pontoon bridge and flosilo a float or buoy. For a boat merely lying on the water rather than drifting, most writers prefer naĝi or resti sur la akvo.