This is an ongoing project: every time I go on scubadiving adventure I try to draw underwater and post my sketches online.
I use all sorts of pencils on a "rite-in-the-rain" paper.
You can read all my reports about drawing under water (while scubadiving) on my blog:
My underwater drawing experience was featured on urban sketchers blog with more details about materials and techniques. You can read it at urbansketchers.com
You can see all images from my underwater sketchbooks in