Above is an image of an unknown plant. It grows without any care.
This is a water-primrose Ludwigia, probably Ludwigia peploides but I am not sure about the species (also because you didn't provide a location). Luwigia peploides and L. grandiflora are native to South america. However, in the rest of the world they are considered invasive weeds. In the European union L. peploides and L. grandiflora are considered "Invasive alien species of Union concern" This means EU member states should take appropriate action against these species.
Water-primrose has a high variation in the shape of the leaves, especially between vegetative and flowering sprouts. On the picture in the question the leaves are almost round. However, on the ground just left of the hand, there are lanceolate leaves, almost willow-shaped. This is the same species and the reason it is also called floating primrose-willow.