For pontoons take a bunch of large plastic soda bottles and glue them into a cylinder shape then fiberglass over it and be sure to provide attachment points for your deck of plywood. Nobody said I was Noah.

2 6mil plastic bags, stuffed inside one another, and filled with packing peanuts.

Make 7 of them and tie a knot in the end of each one.
Tie those knots together with rope.
You will have a flower shaped raft.
I did it in 1996 when I got trapped in by a flood.
It got me 250 feet and I only got my feet wet.

Use your brain.

In any such situation, you will not be assured of having anything. There is no one good answer.

That last post about the packing peanuts was awesome, if you can find some thrown-out packing an plastic bags lying around. If you're in an area where there are plenty of vines and dead logs, make a raft out of those. If the water is big enough to justify it, and you have enough time, make a kayak. If there are old houses around, you can even make a pretty good makeshift raft by tearing out some of the lumber and using it.

Sometimes you might have to swim. Sometimes you might have to wade through it. Sometimes you can find a natural crossing. Sometimes, you might have to go around.

The real trick is not having a blueprint in your head, but in being able to size up the situation and find the best solution. Do you have to cross here? How much time do you have? What can you use that's accessible? Will the fire you use to make that kayak be seen by unfriendly forces? Can you climb across (using overhanging trees for instance)? All these variables change constantly with each new challenge and it is up to you to find the best course of action.

Of course, you could become familiar with the countryside where you will be escaping through and try to avoid large problems in the first place.

TheRedneck

Hmm, all of these ideas sound very good for a raft! But I think that a inflatable raft would be easy to carry around in a backpack. Also I am pretty sure that it wouldn't be to difficult to cut down a few trees and tie them together(if you have the right tools that is.)

A friend of mines brother died of hypothermia trying to swim across a river. Just a warning for those who say learn to swim. I can swim quite well, but if my muscles freeze up because the water is to cold, I'm screwed.