The second time I swam there (it's been a hot, hot day) was at low tide. The pool was calm and I spent more time diving under the water than I did swimming lengths. I love all the rock pool life - the different seaweeds, the small fish, the anemones, the sea urchins and the starfish. While I was over by the wall at the deepest part of the pool, I realised that someone had built a small cairn underwater. It was only five flat rocks tall, but unmistakably a cairn. I couldn't resist diving down and finding a small, flat white stone to add to it. Some time later, when I was shivering and ready to leave the water, I found a small black stone at the bottom of the pool and placed that on the cairn as well.
Later I started thinking about Richard Long and his art made while walking across landscapes. It should be possible to do something similar underwater, in a cove or a bay perhaps, as you swam along or across it.