Anhingas are one of my favorite birds, and we saw plenty of them during our trip last month. The video above shows one we disturbed during one of our walks, and that awful croaking noise is his vocalization.
Like cormorants, anhingas lack the waterproofing that other waterbirds possess. When they dive underwater to hunt, their feathers become soaked through. This and their particularly dense bones (for birds) helps them remain submerged without effort, allowing them to hunt in shallow areas and to stalk their prey. When they surface, their bodies remain underwater with only their long necks sticking up - their sinuous movements make them look like snakes in the water, which earned them the name "Snake Bird." Getting video of this is harder than I expected. We managed to get the video below, but you have to hang in there - the bird pops up twice, once around 20 seconds and once near the end.
Because they get so wet, their time underwater is limited by heat loss. They spend an inordinate amount of time drying off and sunning themselves - the main reason you won't find these birds in Seattle! This female is drying off after a swim.
And last, here's a video of an anhinga dealing with a fish slightly too big and alive for it to swallow: