Just read through 40 comments here.
This nutcase is not saying he’d save 30k gallons PER shower. He’s estimating that’s how much he will save by not taking a shower for over 120 days.
That estimate is still a mind-blow, as it would be an average of close to 125 gallons per shower.
When on my carrier Med cruise (1961), we sometimes had water shortages. We were only allowed to turn on water to wet ourselves, then turn water off. Soap everything and turn water on again to rinse. The shower areas each had a Master at Arms (ship’s LEOs) in the area to ensure compliance. I’d get a total of maybe 6 minutes of water.
According to Creighton, he believes that 30,000 gallons of water is saved for every shower that he doesn’t take during the challenge.
That is what the article states.