beach dunes pools sand Sandbar tidal tide water blue gray sea Stingrays swimming white sand aqua white Barmouth boats coast estuary forest Gwynedd harbour hills hulls low masts moored mudflat wales welsh yachts agitated battle choppy coastline current Devon England english exe flow Great Britain natural nature river turbulent waves bare exposed green landscape Mawddach mudflats peaceful sandbars seashells tranquil view channel guidance guide navigation sign warning birds colors colours dune-flora grass grasses greenery growing growth gulls levels low tide ocean ocean edge ocean waterline pelicans plants receded receding receding tide rough salt-resistant sand-dunes scrub seabirds seagulls tides vegetation waterbirds waterline flora