Rock Bottom

You know how people use the term 'rock bottom' to mean when you've hit as low as you can possibly go. I bet that expression comes from digging. Like when you dig deep enough, you hit rock, or magma, or Satan's lair, or whatever it is down near the earth's core.

But if you've ever been to the beach, you'd know that you can't hit rock bottom there. You dig and dig and dig, and all you get is water and sand in your bathing suit. So I think it should be appropriate to interchange 'rock bottom' with 'wet bottom' for those beach-dwellers.

But then again, in a different context 'wet bottom' is not appropriate.