I dove for years and always needed to save money...if you want really cheap and good 'slates' for 'underwater' use.....take a piece of plexiglass and rough it up with sandpaper so that you can no longer see through it. It is now usuable as a slate. Pencil erases off it with no problem by either eraser or by rubbing with sand (field expediant-works well while underwater). Another nice thing is that you can custom make your slate. And if you know of someone who has 'scraps' you can get it for free. I use to give these away to students as I had a free supply of scrap plexiglass.

Hope this helps someone.