I know a few people have Rhino liner or something similar (basically spray-on bedliner for pickups) that is platic/rubber and would make a great, tough, waterproof, sound dampening material. Plus, it comes in whatever color and you can chuck your floormats after that!
'Brown bread,' Dynamat, etc, have also been used. I happen to have gotten a good deal on Dynamat so I have to install that, but I also have some "roll-your-own" truck bed liner as well.
Just some ideas HTH.
Davy