You're saying the entire indoor volume of the "superloo" gets flooded with water, like a sinking submarine? Don't be absurd. That'd be a monumental waste of water and it wouldn't even clean that well.
And "the entire chamber being -sprayed- with liquid" plus "I felt like I was drowning" could easily get misquoted as well (or misremembered, 20 years ago is certainly more than long enough for me to screw up details).
So my guess is that what actually happened is something of that ilk.