Another short riddle, this one about something that travels over and under water with its head always down. It rewards a slow, puzzled reading before the answer clicks.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
Over the water,
And under the water,
And always with its head down.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition