I would just like to add that even if your packs definitely float, they may not float indefinately. The flotation built into most canoes is just a bit more than it takes to float the canoe -  a few waterlogged packs lashed in may send the whole package to the bottom. Often a canoe roll is due to heavy waves - waves that will batter the canoe and packs when it all goes wrong. So, even though I know that my packs will float (for a while), I never lash them in on flat water. Losing the packs is not as big of an issue as losing the canoe for obvious reasons. On a river, where you will probably be able to recover the mess downstream it makes a little more sense to lash the packs in.

The type of clothing you wear will likely make a big difference in how long you can survive in colder water - dress appropriately.

As others have mentioned, I always have a fanny pack (I call is a splash pack) attached to my body with the stuff I want handy for the trip and the stuff I want with me if everything else is lost.

-aj