The metaphor of ‘the sea of faith’ comes from Matthew Arnold’s poem Dover Beach (1867) and has been explored subsequently by a number of theologians, notably Don Cupitt in his BBC TV series of the same name and the Sea of Faith Network founded to explore his ideas. This paper explores how the image of the sea functions in Arnold’s poem and his The Forsaken Merman before arguing that R. S. Thomas’s use of the sea in his poem Tidal provides a better theological metaphor than Arnold’s.