The airport at Barra, Scotland uses runways in the beach:
The beach is set out with three runways in a triangle, marked by permanent wooden poles at their ends, in directions 07/25, 11/29, 15/33. This almost always allows the Twin Otters that serve the airport to land into the wind. At high tide these runways are under the sea; flight times vary with the tide. Emergency flights occasionally operate at night from the airport, with vehicle lights used to illuminate the runway and reflective strips laid on to the beach.
(via this entertaining video about visiting Britain’s westernmost bus stop)