I've put plants in the shower for various reasons, plain water works fine. I only do it if I've used Neem oil or other soap based insect controls. Other than that, not going to worry about a little dirt. An old saying goes: You eat a peck of dirt before you die. And I'd worry more about weed I'd bought from someone else than my own, people do dirty, nasty things and think its funny......
I didn't think sorry @olderthandirt got it.
I gave that a read but I was wondering if there was other ways, could I get away with using just water?