This is an obvious fake. Nice try Indy.
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.
Keep lying to yourself Indy. We all know you can't swim.