What plant is that?

The October Mystery Plant is a shallow-water native plant found on sandy or mucky shores. Its dense white button-shaped flower clusters, only 1/8 to 3/8″ across, appear in July to September. The individual flowers are indistinguishable. The single stem is a hollow stalk which is slightly twisted with prominent edges. From a basal rosette this stem rises 4-9″ above the water topped by a flower cluster.