I am developing a culture for a species that lives in freshwater. They are solidly stone/bone-age, so no issues with "how do you develop metallurgy underwater?" and so on, but I would like them to be able to develop writing.
Carving rock is possible, but that seems overly cumbersome to become a normal method of writing for general literacy. And ink is a non-starter. So, I am considering a system like cuneiform, intended to be pressed into soft tablets. But while clay is easy to find on a river or lake bed, clay tablets don't exactly hold together well underwater!
So, is there a natural or easy-to-make substance that could substitute to allow for underwater cuneiform? Modern oil and polymer-based sculpting clays ought to work, but would a stone-age society be able to produce something like that?