Waterproof Compression Backpack packing requires the use of trash bag, preferable a durable trash bag. Make sure there are no holes in it. This method of packing not only it waterproof all your gears in the backpack, it also creates more storage space in the Backpack.
Why This Idea
This idea came to me when many years ago, we had a bad experience on Tioman Island, West Malaysia. We had so called rain proof Backpacks. During the off-beaten trail along the coast of the island, we had to back track when the tide came up. Still then, we had to jump into the sea to clear the obstacle. Threw our backpacks into the water and swam with it until we realised the backpacks were taking in water and sunk, nearly drown and it was a close call. We had a hard time pulling up the Backpacks from the cliff's edge as it were all filled with seawater. All our gears were soaking wet including the camera.
How It Can Be Done
I have seen Taiwanese River Tracers, in their backpack they would purchased another waterproof bag such as Sea-Line and they do not come cheap.
Problem with Hiking Backpacks used for such activity is that they will still take in water as they do not have any drainage holes at the bottom unlike Military Backpacks which comes with drainage holes. I would make a hole to drain the water if I am doing such activity but to make a hole on an expensive Backpack, I rather invest in Military packs.