Have a look in toy shops - I think they sometimes sell slates a lot cheaper.
There are things like this: http://www.synergy-group.co.uk/maths13.html
Although they are a bit thick to take diving I think.
My guess is that any plastic you buy will be fairly waterproof 
__________________
When the mariner has been tossed for many days in thick weather, and on an unknown sea, he naturally avails himself of the first pause in the storm, the earliest glance of the sun, to take his latitude, and ascertain how far the elements have driven him from his true course. Let us imitate this prudence, and, before we float farther on the waves of this debate, refer to the point from which we departed, that we may at least be able to conjecture where we now are. |