Great ideas in this thread.
Here's a No bucks project.
I had an old inflatable that was beyond repair. It leaked air and the bottom had come almost completely unglued. I don't really need a dinghy right now but I did have a need for a work raft to get around the boat when she's on the lift.
So, I started saving up plastic bottles. It took a few months for me and a couple of friends to accumulate enough.
made a few slits in the dinghy.
Then stuffed it as full as I could.
Then I sewed up the slits, smeared some liquid nails over the seams, cut out the transom and lashed the pontoons together with the old floor on top.
I tried inserting a little expanding foam to firm things up, but found it really wasn't necessary for my purposes. Besides, I didn't want to spend any money on the project.
Surprisingly, it actually works pretty well. I don't have to worry about scraping it on pilings or over the seawall, and I didn't have to haul it to the dump.