This is a white wildflower that grows in wet ditches and at the edges of ponds.
Duck potato
It got the common name of Duck potato because its tubers are an edible source of food, supposedly for ducks.
More officially, the common name now is Grassy Arrowhead (Sagittaria graminea) because of its narrow leaves.
Flowers in the Sagittaria genus typically have three white petals surrounding a tight bundle of pistils or stamens.