I thought the archers were friendly at first. Then noticed one is pointing their bow down towards the swimmers. The swimmer without a goatskin looked to me to have an arrow (or two?) in them that would have come from that direction. Detail was hard for me to make out though.
(I'm purposefully arguing only pro diving, in case that's not clear.)
Could also be the scene of a siege, the swimmers trying to traverse a river with heavy currents, indicated by the swirls in the water.
In that scenario the archers would give them covering fire from the defending soldiers in the castle structure.
This could also explain the archers wearing different helmets: They belong to different sides in the conflict.