Thank you for your comment!

baptízō seems to be the correct word for Baptism and it means to dip (as you said) but also to submerge and immerse - http://biblehub.com/greek/907.htm. Early church history (before Nicean 1) also shows that full immersion was the overwhelming norm - the only exceptions I can find are when people were too sick for full immersion. Why did we deviate from the practice that Jesus underwent and how the Apostles baptized? Likely because we decided that we would prefer to do things our way and that our logic was better than that of the Apostles.

The idea of a separate body and soul is not something that is found in the Bible in the terms you are describing. It is more of an Ancient Greek concept that was probably adapted later in the Church. More often, the Hebrews viewed the two as integrated. Our eternal life is portrayed in physical terms rather than us existing as ghosts living in the clouds. I think that it overcomplicates the issue and is human hubris to try and project our conceptions onto the word rather than let the word stand on its own. Drawing a distinction how baptism works on the soul does just this. The text is clear that Baptism washes us from our sins. History shows how it was overwhelmingly practiced in the early church. Why do we need to reinterpret this?

Tertullian's example of fish seems apt - but I prefer Paul's words on us being the body and Jesus being the head. We cannot have life apart from Jesus and he is the water that satisfies all thirst (as he told Samaritan woman at the well). Baptism is a transition point to our new life just like birth (it is literally our birth into Christ where we receive the Holy Spirit. On this point - I feel there is another needless complication - Baptism specifically promises us the gift of the Holy Spirit living in us, NOT the gift of sanctified water that lives in us.