Aka Cape Pond Weed, Water Hawthorne, Vleikos Aponogeton distachyos (Aponogetonaceae).
The waterblommetjie is one of South Africa's most famous edible plants. It has a long history of use by the indigenous Khoikhoi, both medicinally and as a nutritious food. The name means "little water flower" and though it's an edible flower, it's not the sort that is used to garnish a salad.
This one is hardy enough to be used in the most proud of South African stews, the waterblommetjiebredie.