For the last few years, I used ziplock bags to keep stuff dry inside my day pack. Sometimes that worked, sometimes it did not. When I am after smallies, I will routinely wade up to chest high crossing the flat stretches of some of the rivers around here. The ziplocks just were not built for total submersion.
Part of the problem was that the water did not drain out fast enough when I emerged on the other side. So, I got my grommet kit and slammed a few holes in the bottom - one in each compartment to allow the water to escape.
To solve the leakage problem, I stumbled across some very small dry bags in the camping section of WalMart. They cost less than 10 bucks and work like a charm. They are small enough to fit in the different compartments of the day pack, yet big enough to hold my first aid stuff, basic survival gear, etc.
If you wade light - just using a vest - you may want to grab one of these to put your lunch in so you avoid eating "stream soup".