After careful analysis I agree with my collogue above. There was a distinct moment where he made the switch and it is in fact not Indy swimming. I also took the liberty to interview multiple beach staff members who did confirm that Indy was laying on the ground and had planned to have a body double switch as that double was walking in tandem with the person who had concealed his action of laying down.If you look closely at 46 to 47 second mark, Indy walks past a man wearing a black shirt and is momentarily out of sight. I believe during this lapse in view there was a switch that occurred. Following the switch, Indy laid flat on the grass as to be out of shot and his friend who could actually swim took his place since they were so far away now that details of who was actually swimming would be obscured.