Ardea herodias occidentalis
Once considered a separate species, this bird is now known to be a white morph of the Great Blue Heron. It is found only in extreme South Florida and the Florida Keys. It's difficult to distinguish from the Great Egret - clues are the larger size, the yellow legs (not shown in this photo, but we could see them when it flew overhead), and the thicker beak, darker where it meets the head.
This photo may not be especially helpful for others trying to learn to ID these birds, but I thought it was worth submitting to document the location of the spotting.