The "Cock lake" is not a lake at all, but a large rivulet formed by water flowing out of the sands into a gully, after the ebbing tide has exposed the flats. In this paragraph water is flowing back in from the rising tide: "In long lassoes from the Cock lake the water flowed full, covering greengoldenly lagoons of sand, rising, flowing. My ashplant will float away."
The photograph at right is posted on Dara Connolly's blog, which contains images from a walk he took on Sandymount Strand. He writes: "One thing I learned in recent weeks is that the sand is not at its driest at the time when the tide is lowest. The recently exposed sand flats hold a lot of standing water in ripples and pools, and it drains off into channels which resemble streams or rivers crossing the beach. The largest of the channels is called 'Cockle Lake' on the charts, or as Joyce would have it 'Cock Lake'. The sand continues to dry out for several more hours until eventually inundated by the returning tide."
"Cock Lake" was not Joyce's invention. That name can be seen on some contemporary maps, including the one at right. But Connolly may nevertheless be right that "Joyce would have it" this way, since Stephen urinates in this paragraph, his urine mingling with the swirling waters of the rivulet.