This is a relatively small duck, with striking black and white head and breast in the male, as pictured. Both the male and female have a crest.
These birds were spotted in the J.N. Ding Darling National Wildlife Refuge on Sanibel Island off the west coast of Florida. They were feeding off the shallow bottom, in a mangrove area and saltwater lagoon. They were diving in shallow water, feeding on crustaceans (see 4th photo above). According to the local naturalist, these ducks are relatively rare in the area, not often spotted.
Hooded Mergansers, according to online sources, find their prey underwater by sight. They have an extra eyelid, a “nictitating membrane,” which is transparent and helps protect the eye during swimming, similar to a pair of goggles. Also, Hooded Merganser ducklings leave their nest cavity within 24 hours of hatching. First, their mother checks the area around the nest and calls to the nestlings from ground level. The newborns scramble out of the nest and then flutter to the ground, which may be 50 feet or more below them. In some cases they have to walk half a mile or more with their mother to the nearest body of water. Source...Cornell Lab of Ornithology...https://www.allaboutbirds.org/guide/Hooded_Merganser/lifehistory