The big concern with enjoying clay, mud, or anything sticky in your bathtub is blocking the drain. I've been playing with clay in my bathtub for many years (see my profile's pictures and videos) without incident, but have had a few close calls which prompted the most simple of solutions.

A few times, my feet have gotten so thickly coated with clay that when I put my foot on the bathplug, the type that just sits in the hole and you twist a knob to raise it to drain the bathwater, the plug has stuck to the mud on my foot and lifted straight out, exposing the drain and the possibility of it accidently filling with thick clay!

The simple solution to this has been a large universal silicon plug that covers the entire plughole. Whilst being a plug in its own right, I put this over the existing plug for two layers of security. Occasionally, it will slide out of position if pushed with enough force, but it is easily pushed back into place, and I've never had a situation where the drain has been exposed since. It's been one of those trivial purchases off Amazon that has been more than its worth in gold.

(Second post sharing the knowledge of how to WAM successfully from what I have learnt over the years)