Really true. Of course any production company or studio doing anything like that would obviously have a shower on hand (you need warm water to mix gunge effectively) - and I've done several shoots and sessions so I know that first hand, including having run all the hot water out of a studio at a large shoot so I had to try to clean golden syrup out of my hair with cold water, which let me tell you doesn't really work very well (thank heavens for the hotel showers being wonderful, and the hotel being right around the corner from the venue).
But my first time in a gunge tank, as described above, was a charity thing at a local outdoor fayre - and they didn't even have buckets of water or a hose to clean up. They had no plan, I was one of only two people to volunteer (and the other chickened out), and it was simply a "Um, hi, can I volunteer to go in the gunge tank?" "...uhh, I think we need like a minimum £20 donation?" - okay, no problem, here's £20 - "...alright, uh, come back here at 4pm then and we'll gunge you" - and I did, and they did.
Fortunately I thought ahead for that possibility, having seen "gunge tank" advertised on the flyer (and almost not believing my luck, and being well aware if it was possible there was no way I wouldn't volunteer to get gunged!) and had a bag to put my keys and stuff in so they didn't get soaked.
It was also by sheer coincidence the hottest day of the year, so the gunge being cold was a pure blessing. My friends there were a little reluctant to return my offers of a hug afterwards, and it was hilarious that while trudging home in a gloriously messy daze, people were very nearly walking into me on their way there and then at the last minute noticing all the slime dripping off me, and suddenly diverting and giving me a very wide berth!