I will try, but this will not be official Church teaching on the matter just as a disclaimer.
Here, St. Peter appears to be alluding to the water used in Baptism, as he alluded to the water of the Great Flood previously.
He is saying the water used in Baptism is not used to wash away dirt, rather to wash away Original Sin, and thus, is required for salvation.
Now, I believe a more important point you should address is the point Petrosius made previously, before your discussion with him got side tracked into the dubious nature of Mark 16:9-20, which is, "Where in Scripture does it say that the Scriptures are sufficient for salvation?"
You also said in your reply, "Some of this will have to wait until later", so I would hope that question of Petrosius' will be addressed in a later post of yours.
Regards,
yes ... I will have to answer that later when I have more time.
Concerning the Peter passage, the simple fact that you mention "allusion" means that the passage is not as straightforward as you imagine.
If Peter is saying baptism is required for salvation, how can you possibly harmonize the Romans 10 passage (and an absolutely huge array of other passages) that do not mention baptism at all?
This is exactly what I mean about using a difficult passage to support doctrine when there are many more passages that are straightforward. If one passage does not support another, you have one of the interpretations wrong. There can be no contradications in scripture.
Peter cannot say one thing and Paul another ...