In 2 Nephi 31:13, Nephi says that people must act "with real intent, repenting of your sins, witnessing unto the Father that ye are willing to take upon you the name of Christ, by baptism." How is baptism a way of witnessing to God that we are willing to keep his commandments? Why is that an act of covenant making? It is because it is a symbolic reenactment of the atoning death and the resurrection of Jesus. It is also symbolic of our own spiritual rebirth as sons and daughters unto Christ.
Today, most people are baptized in a domesticated font in a stake center where it is tiled, perfectly safe, and the water is still and usually warm. In ancient Israel, however, when you were immersed for purification purposes, the water had to be living water—running water—and it was cold, sometimes very cold. Standing water was, by definition, impure. Moreover, Lehi had taught that Jesus would be baptized in the flowing water at Bethabara, the place of crossing the Jordan River (1 Nephi 10:9). That place would also have symbolized risking your life in the river’s current as well as crossing over from one place in life into a new covenant land of promise. Likewise, the early pioneers were all baptized in rivers or oceans or places like that. Most of these people, in ancient or in more recent times, did not know how to swim very well. For them, going into the water could be a frightening thing. In fact, in ancient law submitting yourself to the "river ordeal" was one way of establishing in court that you were telling the truth. Thus, witnessing to God that you are willing to risk your life by going into the water seems to have been a symbol of serious commitment, much more than we usually think of it as today.
This is especially powerful when one considers the universal ancient belief, that under the waters were all types of evil spirits. In the ocean you have the death monster that swallows Jonah, and all of these evil, unknown creatures that are there. The waters, rivers, and lakes were thought to be orifices entering into the underworld. With this understanding, the ordinance of baptism by immersion profoundly showed that you were willing to go down even into the depths of the underworld, and there, through the power of Christ, be brought up out of that water, thereby overcoming all of the evil in the world. By being baptized, you descend below all things and go beneath the world so that you may ascend up above the world. It is a very powerful symbol.