I went to the woodworking show in Seattle last weekend and I bought a 3" sanding pad and a bunch of 80 grit discs to sand the inside of the kayak. I bought them from Peach Tree Woodworking supply.

Here's a similar picture, but mine looks different. The foam on mine is a lot thicker. It has a solid disc with a shaft where it attaches to a drill motor, it has an approx 1" thick piece of conical shaped foam glued onto the solid disc, and a velcro pad glued onto the foam. They are made to sand the inside of bowls and are quite flexible. The theory is that it should work great to clean out the inside of a kayak. I haven't tried mine on the kayak yet, but when I do I'll tell you how it works out.