You guys rock.
Venetian blind material. I've got the cheapest white plastic venetian blinds Home Depot sells, just wrote on a slat with pencil. Works great. (All I had to do was turn around....)
Nalgene Plastic Paper looks like great stuff. I think it's for note taking in rough lab conditions, (acid proof) but no pencil.

As a teacher I've got grease pencils all over the place (overhead projectors and administrators aren't allowed to use anything sharp or permanent), I'd rather use a pencil.

Only problem with the Venetian Blind material--aside from the fact they're not really for kayakers*, they're really narrow.

I think I'll go with a Rite in the Rain notebook. Then I won't have to transfer my trip details and wash it after every trip. 56 pages , 2 sides Amazon 3 1/2" x 5" $6 free shipping.

*Maybe I can find wider slates on the ...... Gondola Building Forum..... clunk!