I think river otters are pretty adaptable... we have them in our gully at the beach house and they go out and hunt in salt water. They come up on the dock to eat their catch in the pre-dawn early morning.
Ours isn't always black like that, usually more green and the bone and shell bits are white. I don't know what that guy's been eating.
Well, there are otters in the Everglades for sure, which are pretty close to where I see the droppings, probably 10 miles to the edge.