Divers on a shot line.
This particular shot line connects the water surface with the wreck of the Thesis in the Sound of Mull, Scotland

After a dive divers must ascend to the surface slowly to prevent air bubbles forming and expanding in their blood (causing Decompression Illness commonly known as The Bends). After longer and/or deeper dives they often have to pause for longer periods of time to allow these dissolved gasses in the blood to off-gass more slowly. This often causes log-jams on the shot line after a dive. This is exactly what is going on in this picture.