I spent a morning looking at the ebb and flow of the water and the foam produced by air and water mixing. This was at the time when the tide was going out.
Later I join threaded polystyrene balls to the scene. The ends of the thread are fixed to stones, so the balls can't escape.
The stringed balls float off as if they were part of the water-foam. For about an hour I watch the water recede. Its movement changes with the shape of the ground and the foam disappears gradually - together with the balls...!
Minutes later - it seems like an eternity - when the water has receded further, I spot one ball hiding among the shingle. The rest of the balls - still threaded - are buried up to one foot underground following the lead of the stone the thread was fixed to.
By the way: Polystyrene consists 98% of air, is fully recyclable and environmental friendly though it isn't biodegradable. Microscopic images of this material look very similar to the sea water foam I'm just looking at.