I can see a big problem occurring already in that your friend will try to retort that “Catholics claim that Baptism guarantees salvation,” which the Catholic Church does not and never has taught. When using the word, “save,” “saves” or “saved,” with a Protestant, you must realize that they have a VERY different understanding of the entire concept of salvation, and the meaning of those terms. Many Protestants believe that “saved” is a result of a one-time event of “accepting Christ,” and will therefore associate THEIR twisted and distorted meaning of the term with what you(and St. Peter) are claiming about Baptism.
In other words, no offense, but I think you are barking up the wrong tree, if you will, by using 1 Peter 3:20-22 as a proof text, because you will essentially be speaking two different languages on that verse, when talking to a Protestant. I would instead recommend, if your goal is to prove that Baptism is regenerative, you should use Matthew 3:11, Mark 1:8, Luke 3:16, John 1:33, Acts 1:5, and Acts 11:16. All those verses make a distinction between the Baptism that John the Baptist was doing, and the Sacrament of Baptism that Jesus Christ initiated, as recorded in Matthew 28:19. Many (but not all) Protestant denominations have resorted back to the Baptism of John, denying The Holy Spirit’s role in the Sacrament. You may then follow up with a question and ask your Protestant friend, “What is the difference between the Baptism that John the Baptist was performing, and the Baptism that your church is performing?” If your friend fails to provide an answer, then you can explain to him/her what the difference is between the Baptism of John, and the Baptism that Jesus Christ initiated, which is what the Catholic Church follows as a Sacrament. You have the six proof text Scriptures I listed above to show that there IS in fact a difference. If your friend belongs to a church that claims, “Baptism is merely an outward sign of an inward commitment,” then you can point out to your friend that his/her church has gone back to John’s Baptism. If his/her church goes one step further and attacks Catholic Baptism, then you can respond saying that his/her church not only rejects a commandment that comes directly from Jesus Christ(Matthew 28:19), but disdains it as well.