Here is my thought on your plan. Let me preface this with the fact that I don't know what I'm talking about.

I would think that getting the rear of the boat to float to the surface is going to be a challenge. If you fill the bow with a bunch of stuff that floats it's going to bring the bow up but leave the stearn submerged. This wouldn't give you a chance to pump the water out.

I would also thing that trashbags would be hard to fill and keep filled underwater but I'm not sure.

I would think the best thing to use would be an air compressor and a bunch of old truck intertubes. You can cut the truck intertubes in half and seal both sides for a long, straight float. I would also think that a bunch of good line to tie the intertubes to the boat in the right places is going to come in handy.

I'm thinking with dive equipment, freinds, line, and inflatables you will be successfull. It sounds like fun.

Or you could get a surplus shipment of beachballs at Big Lots and have fun putting them in the hull.

Mike Hill
www.stlouiscats.com