Fanwort & Hornwort
Fanwort
Fanwort is a fully aquatic true plant. It grows steadily through the warm months, and can choke-out large areas, then dies back in the winter. Fanwort produces tiny flowers above the surface.
Hornwort
Hornwort is very similar to Fanwort. In fact, unless you are a botanist, it might as well be the same thing. Hornwort produces tiny flowers underwater, an easy way to tell them apart.
Fanwort is the more common of the two, if you are paying attention. Both grow almost to the surface, but are not usually a problem for paddling. As with Water Lilies, I have tried to take Fanwort for the aquarium, but it just dies. My guess is it needs full sun and nothing less will do.