Found this rare picture online showing that a man was being baptized. Judging by the clothing of the people and other details (the man being baptized seemed to still have a braided ponytail), it looks like it might have been around the 1920s.
Water is not visible in the pool, but we can still observe some interesting details. The person performing the baptism doesn’t seem to be in the clothing of a clergy – not sure if this means baptism were performed during that time not by official clergymen but by local assistants or others.
There was a large crowd watching, demonstrating a good tactic perhaps on the part of the church to do this in an area that would attract large audience and therefore gain more awareness of Christianity. There were many children, and influencing them is always important.
The fact that a crowd had gathered to watch the baptism may show that it was still something very rare at that time, at least in the place where this happened. The people seemed to be hard-working laborers of the less fortunate type, which could prove extra evidence of the spread of Christianity among the poor.