Not quite seaside -> inland, but the area around Ely is still called the Isle of Ely because Ely used to be an island surrounded by marshy fenland, often floode

Not quite seaside -> inland, but the area around Ely is still called the Isle of Ely because Ely used to be an island surrounded by marshy fenland, often flooded by the wash from the North Sea. The fens were drained in the 17th century onwards with the help of some Dutch engineering and Ely is now just a hill.