Floating a Backpack Across a River

I'm hiking the Otter Trail in South Africa next year. We have to float our backpacks and swim across the river at one point. My question is what's the best way to do this: Tie up the whole backpack inside a plastic garbage bag; or put all my stuff in plastic bags inside the pack? And maybe the bigger question is, If I drown while I'm crossing, does it really matter about my stuff?