It's an identification with Christ. Water baptism is a public, symbolic way for us to identify with Christ in his death, burial and resurrection. We tell the world that we belong to Jesus and we are under new ownership. As a result, we are new people on the inside. Going down into the water and coming up again are powerful symbols of death and resurrection.
Baptism is an outward ceremony that points to an inner change.
If the inner change has not happened or we have not accepted Christ, baptism has no more meaning than going for a swim or having a wash. This means we cannot be lone Christians doing our own thing. Just as you cannot be a successful football player if all you do is play alone in your garden, you cannot effectively live the Christian live if you do not join with others, learn from them and work with them. Belonging to Jesus means belonging to a local church and sharing in their life of worship, prayer and work in order to see society changed by the power of the Good News of Jesus Christ. It symbolises washing away past sins.