Hydrangea arborescens; Hydrangeaceae (hydrangea family; older sources place it in the Saxifragaceae)
I was lucky to find this particular plant; it isn’t rare, but it is unusual to find a specimen with large clusters of the showy sterile flowers (actually bracts). More typically in the wild you’d see an inflorescence of much smaller (and less spectacular) fertile flowers, with just a few of the sterile flowers around the edges. Wild hydrangea is a small understory shrub that likes moist but well-drained soils. It is listed as endangered in Florida and New York. This one was found on the Cabin John Trail, not very far from the water.