Darter or Snakebird

The darters or snakebirds are mainly tropical waterbirds in the family Anhingidae having a single genus Anhinga. I think this is that bird. The boat driver referred to it as snakebird.

The Anhinga (Anhinga anhinga), sometimes called Snakebird, Darter, American Darter, or Water Turkey, is a water bird of the warmer parts of the Americas. The word anhinga comes from the Brazilian Tupi language and means devil bird or snake bird.

It is a cormorant-like bird with an average body length of 85 cm (33 in), a wingspan of 117 cm (46 in), and a mass of up to 1.35 kg (3.0 lb). It is a dark-plumaged piscivore with a very long neck, and often swims with only the neck above water.

Unlike ducks, the Anhinga is not able to waterproof its feathers using oil produced by the uropygial gland. Consequently, feathers can become waterlogged, making the bird barely buoyant. However, this allows it to dive easily and search for underwater prey, such as fish and amphibians. It can stay down for significant periods.