Pliny the younger recounted a visit he had made to see floating islands in a lake in Italy:
… I have lately been entertained with one of these curiosities, to which I was an entire stranger before. My wife’s grandfather desired I would look over his estate near Ameria. As I was walking over his grounds, I was shewn a lake that lies below them, called Vadimon, about which several very extraordinary things are told. I went up to this lake. It is perfectly circular in form, like a wheel lying on the ground; there is not the least curve or projection of the shore, but all is regular, even and just as if it had been hollowed and cut out by the hand of art. The water is of a clear sky-blue, though with somewhat of a greenish tinge; its smell is sulphurous, and its flavour has medicinal properties, and is deemed of great efficacy in all fractures of the limbs, which it is supposed to heal. Though of but moderate extent, yet the winds have a great effect upon it, throwing it into violent agitation. No vessels are suffered to sail here, as its waters are held sacred; but several floating islands swim about it, covered with reeds and rushes, and with whatever other plants the surrounding marshy ground and the edge itself of the lake produce in greater abundance. Each island has its peculiar shape and size, but the edges of all of them are worn away by their frequent collision with the shore and one another. They are all of the same height and motion; as their respective roots, which are formed like the keel of a boat, may be seen hanging not very far down in the water, and at an equal depth, on whichever side you stand. Sometimes they move in a cluster, and seem to form one entire little continent; sometimes they are dispersed into different quarters by the wind; at other times, when it is calm, they float up and down separately. You may frequently see one of the larger islands sailing along with a lesser joined to it, like a ship with its long boat; or, perhaps, seeming to strive which shall outswim the other: then again they are all driven to the same spot, and by joining themselves to the shore, sometimes on one side and sometimes on the other, lessen or restore the size of the lake in this part or that, accordingly, till at last, uniting in the centre, they restore it to its usual size. The sheep which graze upon the borders of this lake frequently go upon these islands to feed, without perceiving that they have left the shore, until they are alarmed by finding themselves surrounded with water; as though they had been forcibly conveyed and placed there. Afterwards, when the wind drives them back again, they as little perceive their return as their departure.
These floating islands still exist – not those specific ones, but elsewhere around the world. This picture is from Mexico (see more from the source):
Loktak lake in India:
Actually, there’s a whole book about them (that I haven’t read):