As is so often the case, this plant is not a native to the UK, but it originates from South Africa. It is known as Buttonweed (Cotula coronopifolia). Sometimes called Brass Buttons or even Golden Buttons, the name comes from the small, button-shaped flower heads which look like rather the central section of a daisy inflorescence. Here, the Buttonweed can be seen growing in the water, alongside the red stems of Water Mint.
Buttonweed is a perennial and can now be found colonising large parts of the world, spreading itself across the globe particularly in the USA. Not only is it a plant of fresh water marshes and lake margins, it is also salt-tolerant and can be found growing in salt marshes, its fleshy leaves storing fresh water during saline inundation.
Looking in the other direction, away from the Buttonweed, the small, fluffy 'buttons' of mallard ducklings were swimming past, not in the least bit interested in the Buttonweed.