I drew some of my underwater subjects above water and just drew somethings around me too. The thing I always wanted to do and that did not work (again) is collecting all the colors I see on the island - it's an overwhelming task- and when you add it to the overwhelming of drawing everything I see underwater - one of them falls off the drawing board :)
As for the tools: I used the rite-in-the-rain paper and amazingly sleek and indestructible expedition sketchbooks from Field Notes.
Little PVC board for the support was very useful as it allowed me to pack my fish ID cards, sketching tools and magnifying glass all in one place - which is critical for descend and ascend as well as any situations when you need all your attention not on sketching but on your scuba diving skills or buddies. Having everything attached to the support board allowed me to stove it away in my pocket quickly.
I used all sorts of pencils, tried regular wax crayons (not happy with the results) and still believe that plain mechanical pencil is the best. But I lost my favorite one during one of the dives :( After that, I sewed a little wristband which helped me to hold my pencils in one place and was large enough to see if/when it started to float away from me. (I saw my mechanical pencil floating on the surface from the depth of 70 feet but it was just the beginning of the dive and it did not make any sense to drag my buddies up to catch it.