I am in the Balkans and have befriended a refugee who intends to crossing a river as part of their route. They are a very strong swimmer, having practiced it at a competitive level. They want to take their possessions across ideally without these getting wet. I am thinking they might be able to 1) tie a long piece of string to a tree on the near bank, 2) swim across with the loose end, 3) find a rock which fits well in their hand, tie the loose end to it, and throw it over a branch 4) tie the loose end to a tree, 5) swim back across, 6) untie the string at the near bank, 7) clip a waterproof bag such as http://www.iglusport.si/oprema/nahrbtniki-in-torbe/vodotesne-torbe around the string 8) tie a second piece of strong to the bag as a leash 9) tie the first piece of string to another rock, 10) throw it over another branch, 11) tie the first piece of string to a tree, so that it is as taut as possible over the river, 12) pull the waterproof bag with laptop etc inside across the river using the leash.
Is there a better method? I am aware you can keep valuables such as laptops in these bags (as long as you don't submerge them), but I thought suspending it across the river might provide a failsafe.