Concerning water baptism, the Apostle Peter says:
it is a figure or picture of salvation,
it is not a literal washing or cleansing,
it is the response (answer) of a clear conscience.
ref
1 Peter 3:21
Water baptism is considered by many to be a picture of salvation in the same way the bread and wine are pictures of the broken body and shed blood of our Lord. When we partake of the wine and bread we "declare" or "shew" the Lord's death. When we self-immerse or are immersed by another we "declare" that we believe in the death, burial, and resurrection of Jesus and that we have died, were buried, and have risen with Him into a new life.
I realize that not all Apostolics see it this way.