I have used Patagonia's dry bags for years canoeing and rafting. In 2007 my son and I put together a self guided arctic river rafting trip for 10 days. I used bags that were 10 years old (Black Hole model).
Neither river water nor rain penetrated these bags. Everything stayed dry in these bags. They'd probably work well in a Bob too. BTW I always double bag whatever brand name I use on wilderness trips.