I guess the thing I am most drawn to is the reflection of light and clouds on the wet cusp of sand left by the retreating backwash. Like a magic mix of colour and movement. There are the almost mirror like reflections of clouds, or a swirling abstract mix of a long exposure as the backwash retreats.
I usually wait until all light had drained from the sky. And I enjoy seeing the forgotten light that appears near complete darkness, subtle shades. There is also a change in the air at those times, silence and peace and coolness descends on the beach. Sometimes, a sea mist lifts off the backwash into the dunes. The lights of trawlers bobbing out on the horizon. Sooty Oystercatcher piping unseen.