The bags of money were tied underneath the raft so they could not be seen. Tearing into the river rock, they lost a bag and is also why cash was floating beside them when they passed the group of kids on the beach.
Lynn brought her iPhone on a rafting trip without so much as a basic case on it, yet alone waterproofing. She simply kept in in her backpack while on the river.
This entire movie takes place along lakeside waterfront and rivers, but when Victor runs into Millar for the second time, Millar asks for water and chugs the entire bottle, clearly highly dehydrated despite being essentially surrounded by water.
When they get into a scuffle shortly after finding the money, Victor blames Jeff for breaking the strap of his backpack, yet the pack was not involved in the fight. Perhaps the director blocked the scene differently for some takes, and Victor was wearing the pack when Jeff grabbed at him, but in the scene used the pack was just sitting out of the way on the ground.