Nephi is citing a personal vision. It is important to note that in this vision he makes a clear distinction between Father and Son. What he presents is two witnesses (and two very strong witnesses!) to the necessity of repentance and baptism. As with the evidence from the Qumran community, the concept of baptism was to complete the rite of repentance. One comes to the baptism repentant, and seals that repentance and becomes ritually clean after the spiritual reconciliation has been made.
In keeping with Nephi’s theme of following Jesus, he explicitly ties the baptism of his people into the baptism of the Savior. Just as the Savior in fleshly humility followed the will of God, so must we, in those same circumstances, follow the example of the Savior and submit to the Father.