Water baptism by immersion is an outward expression of an inward change, in that when someone gives their life to Christ they are changed from the inside out. Getting water baptised is a way of demonstrating that change.
Water baptism is also a sign of obediance to Christ, as He Himself said it was a requirement, and was water baptised. The bible says in Acts 2:35 “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins”.
Water baptism is also a representation of the death, burial and resurrection of Christ. We go down in the water just as Christ died and in the water we leave our old self and rise up as a new creation in Christ.
In Mark 1:9-10, we continue to read, "And it came to pass in those days that Jesus came from Nazareth of Galilee and was baptized by John in the Jordan. And immediately, coming up out of the water, He saw the heavens parting and the Spirit descending upon Him like a dove."