Sandwich in Kent would be one of the best known.

New Romney? It's riverine, but I think they had to dredge it to keep it alive.

Kenfig in wales.

Dunwich.

Rattray in Scotland. covered in Sand, like Skara Brae maybe?

Faversham may have been, The isle of thanet is no longer an island

Tenterden -once part of the cinque ports. Also Lydd.

I would agree "meaning seaside" was a strech. I think most of these have mixed etymology.

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.

Not what you were asking, but the Adriatic sea, between Italy and the Balkans, takes the name from the town of Adria, which is now 10+ km from the sea (though that is mostly due to the work of the Po river).