you have to wait for it...it lifts its snout out of the water for a breath of fresh air at the end. that dark shadow under the water is a manatee. They are hard to see in water, and usually by this time of year, they are up at the power plant for the warm water. Cold kills them. They are vegetarians and very slow moving gentle creatures. Here is an interesting article about Manatees https://oceana.org/blog/what-you-didn%E2%80%99t-know-about-manatees-honor-manatee-awareness-month