I'm writing a story that takes place entirely underwater, mostly in a deep sea town and shallower city. Both are underwater and populated by beings who can survive underwater.
My thinking is that they send messages to each other by taking a soft clay-like material (let's call it substance x until someone thinks of what it should be) and writing on it, then giving it to a messenger. The messenger takes this wad of substance x with the writing on it and delivers it to the recipient.
When they are done the wad is given back, the letters are smushed away, and it is reused. What material should this be?
It needs to be something that can be made of substances found in the ocean, can be made underwater, doesn't dissolve into the saltwater, lasts a fairly long time, and is inexpensive.
Is there a real-world substance that meets all these requirements, or am I gonna have to make something up?