Hey TimThere can be a case made that water baptism was only for Israel. Israel was to be a nation of priests and water baptism was a requirement for priest. This is why Jesus was water baptized.
If one considers the gospel of the kingdom as a gospel for the nation of Israel to receive the earthly kingdom distinct from the gospel of grace given to Paul mostly for the gentiles, one can also see a difference in baptisms (immersions).
Paul describe "one baptism" for the gospel of grace, that being the immersion of the believer into the body of Christ.
In Acts 2:38 Peter is responding to the question "What should we do?" The baptism of repentance was an established practice that also demonstrated the change of mind (metanoia) that understanding that they had put to death the Messiah meant. The word eis has a broad number of meaning. The AV translates it as "unto" the remission of sins. This may be more accurate as Jesus said that if people did not believe he was the Messiah, they were still in their sins. The Acts 2:38 verse might be saying that the faith of those Peter was answering and demonstrated in repentance and baptism would be unto the forgiveness of sins as well as the door through which they could have the Spirit fall on them.
Water baptism has evolved in meaning, probably due to the fact that it always pointed, and as what it pointed to became more clear, the meaning changed, even from John to the disciples. John's water baptism pointed forward, the disciples water baptism pointed back. Interesting, that Johns water baptism, pointing forward, was for the repentance of sin (Mark 1:4, Acts 19:4), but that could not have been a reality before anyone was placed into Christ at Pentecost, and there was a death, resurrection, and ascension. That's why he said that He baptized into water for repentance of sin, but the One coming after Him would baptize/place into with/by the Holy Spirit.
In Matthew 28:19-20 Jesus commanded them to "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all things that I have commanded you; and lo, I am with you always, even to the end of the age." Amen."
"All the nations" being the point.
I believe that the "one baptism" would be the baptism with the Holy Spirit. And while the purpose of water baptism changed, it's still commanded. The only baptism that can save anyone is that of the Holy Spirit, and the Holy Spirit is received by faith. This is true everywhere in the Bible except the few instances in Acts, and a few passages that things were read into them that were never meant.