Stop and look into any pool formed on the Dorset heathlands and you may well see bog pondweed (Potamogeton polygonifolius) growing. Pondweeds are quite common on still water and here are various species but, as the name suggests, bog pondweed is the one you are most likely to see growing in acid water and that means, in Dorset, on the heaths.
The leaves mainly float on the surface of the water but some occasionally protrude above and others remain submerged. I suspect varying water levels due to rainfall and evaporation may have something to do with this. It can survive if the water disappears completely and you can encounter it on muddy surfaces with no water present. The leaves look dead and over because, although they start green they quickly turn to their natural colour, reddish brown (that probably comes from the chemistry of the water). The leaves are narrow and pointed witch also helps distinguish it from its most common relative, broad-leaved pondweed. The flowers appear on spikes that emerge from the water above the leaves; these too are reddish brown in colour. They look a bit like plantain flowers and have no petals.
This plant can sometimes be found in garden ponds provided the water is acidic enough but it can be quite invasive so I would not seek to try and encourage it in our pond!