In Shaul's letter to the Messianic Hebrews he states that the doctrine of "baptism(s)" (or mikvaot in Hebrew) is one of the foundations of our faith in Messiah. The term mikveh refers to "running or living water." The symbolism indicates that our sins are being washed away and carried away forever. A large natural lake or stream has always been appropriate, but most of the mikvaot used during the early synagogue period were manmade and the rabbis developed several criterion and mechanical means whereby a manmade mikveh could become "living water."
The mikveh was most often done in private with no witnesses present. When done as a declaration of faith and identification with Messiah it is much more common to follow the tradition of Yochanan Ben Zachariah and have a public mikveh .
In my 1.5 hour teaching on The Mikveh - The Doctrine of Baptisms I detail the many "baptisms" referred to in the Brit Chadasha and their relevance to us today, which will be a great benefit to those who want to understand the depth of this practice, which is well over 3000 years old.