I might as well jump in (ha ha, pun intended).
Now I heard that people on the way to the Temple had to bathe to purify themselves as others here have stated, in the ritual sense. So, the physical washing was supposed to remind them of a spiritual washing before they entered the Temple area for worship.
So, John the Baptizer’s “living waters” was in the first instance a reference to the flowing river water, and we’d take that now as foreshadowing Baptism by the Holy Spirit. But, John was emphasizing the proper baptism was one of water and repentence for sins, before going on to the Temple. So, his teaching was deeply prophetic.
As a footnote, when Jesus was baptized He immediately went up out of the water. Why did He do that? I suppose because He didn’t have to linger in the water repenting, which is an important detail that early Christians might have instantly recognized.
And, further, from fundamentalist land, why did He have to be baptized, as He said He must? Well, going back to Noah’s flood, water was a sign of God’s condemnation. And so, Jesus “had” to take on Himself the sign of condemnation. And, that, my friends, is why John says, behold the Lamb of God who takes away the sins of the world. He took on Himself the sign of the condemnation.
And, even further, that’s why evangelicals prefer and insist on immersion, as it symbolizes the same condemnation for our sins and momentary death of the old man/woman in the water, and the emergence of the new man/woman from the waters.
I had the same original question, too, about Baptism, because, according to Strong’s, the word does not occur in the Old Testament. If an old testament word were to be translated ‘baptism’ it would confuse the heck out of us for some time, but it would tip us to the earlier ritual washing.
Baptism is not the banal “sacrament of initiation” It’s about the greatest thing that ever happens in our lives !