The one baptism is spiritual baptism
1Co 12:13 For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.
The only thing water baptism does is get you wet.
The Spirit who had hovered over the waters of the first creation descended then on the Christ as a prelude of the new creation, and the Father revealed Jesus as his "beloved Son."
Mt 3:16-17
| 16And Jesus, when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the Spirit of God descending like a dove, and lighting upon him: 17And lo a voice from heaven, saying, This is my beloved Son, in whom I am well pleased. |
this is linked to what happened when Christ's side was pierced - Jn 19:34
| 34But one of the soldiers with a spear pierced his side, and forthwith came there out blood and water. |
1 Jn 5:6-8
| 6This is he that came by water and blood, even Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. 7For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. |
Note "to be born of water and the Spirit"
Jn 3:5
| 5Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. |
Water is the means through which the Spirit moves -- but you are correct that it is not the only means, only the normative way. One may also be baptised by blood as the martyrs did.
And, as you correctly point out, Baptism brings about this renewal, as Titus 3:5 says baptism brings about this spiritual effect
read Acts 2:38 to find out what baptism is for.
how many baptisms are there according to Paul?