Golden Gate Weir No. 1 as seen in Naples Florida looking upstream
Both dive under the water's surface, i.e. swim, to catch their prey. Usually the clue for me is that anhinga's are found on freshwater bodies of south Florida and comorants in the salt. The catch here is the location. Behind the structure in the background is freshwater and in front where the bird is swimming is the brackish-tasting estuarine ebb and flow side. For that reason, my guess is comorant.
I would flip a coin if it was roosting on the structure itself.