The only rock? What about meerschaum?
Not all meerschaum floats. The meerschaum that does float isn't as buoyant as most pumice.
I know not all meerschaum floats, the point is that pumice is not the only rock that floats.
Not a particularly good idea to attach one's message to pumice if the intended recipient is any distance away - it quickly becomes waterlogged and then sinks....