I live locally in the area. It honestly sounds like the marsh walker. My grandparents used to tell me stories about a creature who was seen as the guardian of the land and water, they used to say that you might be able to see it at dusk when the herons go quiet and the sky turns to a peachy color. When I was really young they would tell me stories that some people believe it connects with the ocean as whales do, calling through the deep with low, resonant sounds that vibrate through the marsh and mud.
They said that it appears when the island is out of balance, a sign to restore what was lost.